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.
What will happen if the two operations above raise? Do we need to catch and mark the build as failed?