Adding initial support for GitHub Apps. (#1738)

* Added authentication using bearer token.

* Added Installation and AccessToken clients.

* Added new clients to Reactive project

* added support for DateTime serialized as FileTime

* added support for StatusEventPayload

* added support for StatusEventPayload

* Added test for StatusEventPayload and fixed serializer to return that event payload type.

* WIP - added ApplicationClient and related Api Urls.

* Continued implementing Installations support.

* Fixing build (WIP)

* fixed build

* added Account property to Installation. prefer nameof(x) over literal "x".

* fixed according to code review.

* fixed build.

* switched Installation ID from int to long.

* added Permissions and Events properties to Installation.

* added documentation to Application and Installation properties in IGitHubClient.

* wip - added tests to new clients

* wip - fix build

* wip - fixed build.

* added InstallationsClient tests.

* added integration test for InstallationsClient.

* changes requested in code review.

* add Get method for App

* Create GitHubApp response model instead of re-using existing Application response model

* add Get method to observable client

* fixed build (both locally and failed test).

* Fixed documentation and added some missing XML docs.

* added DebuggerDisplay to StatusEventPayload

* updated XML docs and added some missing bits. prefer nameof(x) over literal "x".

* Add xml comments to AccessToken response model and use DateTimeOffset rather than DateTime

* Tidy up XmlComments and make consistent across client and observable client and interfaces

* fixup unit tests to independently verify preview header

* Implement GetInstallation method

* revert commits unrelated to GitHubApps - these can be done on a separate PR if required

* this extra authenticator class doesnt appear to be used anywhere

* undo project file change as it doesnt appear to be necessary

* Revert "Merge remote-tracking branch 'remote/GitHubApps' into GitHubApps"

This reverts commit c53cc110b8d807f62fdfeaa7df19e1532d050007, reversing
changes made to 0c9e413d420a4725738644ea5b13af6ec102d456.

* Revert "Revert "Merge remote-tracking branch 'remote/GitHubApps' into GitHubApps""

This reverts commit 02d52b8adf814b6945c60cb59a907a8cd34b1ce7.

* add XmlDoc comments to response models and flesh out installation permissions

* name AcceptHeaders member consistently

* accidentally lost changes to Credentials.cs

* Enhance Intergation test framework to handle GitHubApp settings and discoer tests appropriately
Get code ready for GitHubJWT nuget package but for now just hardcode a JWT in ENV VAR
Add 1 integration test for each method and ensure they are working!

* fixed compiler warnings.

* Added support for Installation=>Id field that arrives in a Pull Request Event payload.

(See the last field in the sample JSON of https://developer.github.com/v3/activity/events/types/#pullrequestevent)

* Change integration test project to netcoreapp2.0 so we can use GitHubJwt nuget package in integration tests

* First cut at some GitHubApp doco

* update mkdocs config

* Moved the Installation property to ActivityPayload, so it's available in all payloads.

This feature is not undocumented, unfortunately, but valid:
https://platform.github.community/t/determining-which-installation-an-event-came-from/539/11

* Split Installation to Installation and InstallationId, and added a comfort method for gaining its AccessToken.

* fixed InstallationId CreateAccessToken to receive IGitHubAppsClient. added (and fixed) docs.

* reverted object-oriented style comfort method and it's docs.

* update all test projects to netcoreapp2.0

* tweak build configs to use 2.0.3 SDK

* also need to update cake frosting build to netcoreapp2.0

* tweak docs some more

* fix convention test failures

* test projects still had some old runtime parts in them!

* travis osx image needs to be at least 10.12 for .NET Core 2.0

* shell script might need the same argument tweak for cake

* more doc tweaks

* Make sure compiler warning output isnt somehow causing Linux and OSX builds to fail

* moar logging for linux/OSX builds

* stop sourcelink on linux/OSX builds to see if that is the problem

* set verbosity to detailed for the dotnet build step

* try new sourcelink and list out remotes

* is travis being weird with git clone?

* SourceLink may be defaulting to true on CI server so explicitly set it as false rather than omitting it

* detailed is a bit too verbose for travis, try normal

* turn sourcelink back on for Linux/OSX

* fix compiler warning

* Try SourceLink.Create.CommandLine instead of SourceLink.Create.GitHub

* CliToolReferences did not update to latest versions

* remove debug origin info

* turn off msbuild output

* go back to SourceLink.Create.GitHub!

* time diff between dev PC and API causes issues if specifying a full 600 second token

* handle extra date format that Installation end point now returns

* field needs to be protected in order to be deserialized

* provide even more buffer for client vs server clock drift

* Update to latest GitHubJwt reference

* go back to SDK 1 since SDK 2 is having sporadic travisCI faliures in TestSourceLink build step

* get appveyor working

* update sourcelink back to latest, and use SDK 1.04 (runtime 1.0.5)

Octokit/IGitHubClient.cs

@@ -38,6 +38,14 @@ namespace Octokit
         /// </remarks>
         IActivitiesClient Activity { get; }
 
+        /// <summary>
+        /// Access GitHub's Application API.
+        /// </summary>
+        /// <remarks>
+        /// Refer to the API documentation for more information: https://developer.github.com/v3/apps/
+        /// </remarks>
+        IGitHubAppsClient GitHubApps { get; }
+
         /// <summary>
         /// Access GitHub's Issue API.
         /// </summary>