Use GitHub status API to report Hound status

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.
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