Added missing methods on IRepositoriesClient

Added unit tests for newly added methods
Added models RepositoryTag, RepositoryLanguage and RepositoryUpdate
Added missing methods on IObservableRepo...

sitoriesClient
Added unit tests for new methods.
Added integration tests for new methods on...

RepositoriesClient
Merge branch 'master' into repositoriesclient

Conflicts:
	Octokit.Reactive/Clients/IObservableRepositoriesClient.cs
	Octokit.Reactive/Clients/ObservableRepositoriesClient.cs
	Octokit/Helpers/ApiUrls.cs
	Octokit/Octokit-Mono.csproj
	Octokit/Octokit-MonoAndroid.csproj
	Octokit/Octokit-Monotouch.csproj
	Octokit/Octokit-netcore45.csproj
Added DebuggerDisplayAttribute to Repository...

Update
Merge pull request #319 from pmacn/repositoriesclient

Implementations of the missing RepositoriesClient methods