Use GitHub status API to report Hound progress

This addresses https://github.com/thoughtbot/hound/issues/503 and https://github.com/thoughtbot/hound/issues/217.

Since GitHub now displays [multiple statuses](https://github.com/blog/1935-see-results-from-all-pull-request-status-checks) it finally makes sense to use the status API to show Hound's progress.
If we start using the string representation ("GithubAPI") the ugprade to RSpec 3.1 will be easier, since 3.1 verifies that the methods exist on the actual class (ala rspec-fire).