Fix way to throw exceptions in async methods
Fix an implementation of Status.Contributors

Create a converter to parse "contributors"
Remove unnecessary statement
Merge pull request #56 from chitoku-k/fix/contributors

Close #54 : Fix an implementation of Status.Contributors
Merge branch 'master' into feature/universal2

Conflicts:
	CoreTweet.Streaming.Reactive.Pcl/Stream.cs
	CoreTweet/Internal/TokensBase.Async.cs
Fix bugs caused by 5021ebeb7c903dc2eee5057c7f29bad7b334394d