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.

![hound status](https://cloud.githubusercontent.com/assets/154463/5476425/96521998-85d6-11e4-97e5-15a28897a779.gif)