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.
@gylaz, I believe the job will fail and will be enqueued to run again.

We should probably make the build idempotent so we can raise and re-enqueue the job without risk of commenting twice or creating multiple build records.