Add support for github statuses

[iphydf]

See https://developer.github.com/v3/repos/statuses/

This is used in pull requests. I need it to make http://git-critique.herokuapp.com/hello/pulls more useful.

[StevenXL]

@phadej I think we can close this issue no?