Change response models 'Url' properties from Uri to string (#1585)

* Add convention test to ensure 'Url' properties are of type string

Closes #1582

* Change 'Url' properties from Uri to string

Global Find/Replace FTW!

* fix compilation errors in the integration tests project

* Extend 'Url' properties type check to request models

* Stick to convention tests naming convention

* Remove unused using directives in models

Changing from `Uri` to `string` means the `using System;`
directive was not needed anymore in some files

* Update exception message wording

* empty commit to trigger a new build - hopefully Travis passes

* add convention test to ensure request models have Uri 'Url' properties

* make request models 'Url' properties Uri

fix typo in convention test name

* revert some request models 'Url' properties as `string`

see https://github.com/octokit/octokit.net/pull/1585#issuecomment-297186728

* Change test so that all model types must have 'Url' properties of type string

 - Filter test input to only get types which have 'Url' properties
 - Merge response and request model types tests into one
 - Unparameterize the exception since we only check for the string type now

* Fix string.Format tokens

If this PR doesn't get rebased, it'll be my wall of shame FOREVER!

* and then it's even more embarrassing when the commit message says rebased but you really meant squashed

* Remove exclusion of `Release` from request models

Octokit/Models/Response/GpgKey.cs

@@ -2,9 +2,6 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Globalization;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
 using Octokit.Internal;
 
 namespace Octokit