Repository invitations changes (#1410)

* add invitations accept header

* create class RepositoryInvitation

* add repository invitations client

* add api urls for invitations

* [WIP]

* add methods to repository invitations client

* add invite method to repo collaborators client

need to add some new overload to post method in apiconnection

* some changes

* add observable client

* add dependings

* add missing observable client

* add missing xml params

* check client

* change repository invitation model

* [WIP] tests

* [WIP] tests; fix overloads for client

* change GetAllForCurrent; suppress message

* some more tests

* [WIP]

* [WIP]

* add collaborator request model

* change return types

change return types for invitation methods. add permission attribute for
repository collaborators invite method.

* add some more tests

* fix xml doc

* check for null arguments

* fix tests

* some fixes from @ryangribble

* add parameterless constructor for RepositoryInvitation

* change setter

* change constructor

* fix merge conflicts

* [WIP] RepositoryInvitationsClientTests

* fix api url xml

* change collaborator request constructor

* change unit tests for collaborator request

* change repocollaboratorsclient

change overloads for add in invite methods to set permissions

* [WIP] integration tests

* add methods for interface

* NotFoundExceptions

* add overload for invite method

* rename repo property

* gramar

* overloads for observable repo collaborators client

* change integration tests

* new integration tests

* add decline test

* add test for accept invitation

Octokit.Reactive/Clients/ObservableRepositoriesClient.cs

@@ -33,6 +33,7 @@ namespace Octokit.Reactive
             Content = new ObservableRepositoryContentsClient(client);
             Merging = new ObservableMergingClient(client);
             Page = new ObservableRepositoryPagesClient(client);
+            Invitation = new ObservableRepositoryInvitationsClient(client);
         }
 
         /// <summary>
@@ -855,5 +856,13 @@ namespace Octokit.Reactive
         /// See the <a href="https://developer.github.com/v3/repos/pages/">Repository Pages API documentation</a> for more information.
         /// </remarks>
         public IObservableRepositoryPagesClient Page { get; private set; }
+
+        /// <summary>
+        /// A client for GitHub's Repository Invitations API.
+        /// </summary>
+        /// <remarks>
+        /// See the <a href="https://developer.github.com/v3/repos/invitations/">Repository Invitations API documentation</a> for more information.
+        /// </remarks>
+        public IObservableRepositoryInvitationsClient Invitation { get; private set; }        
     }
 }