Checks support

By: Piotr Kaminski

I recently hacked in support for GitHub's new Checks API — they should now show up in Reviewable's Checks dropdown along with the usual commit status updates:

Note that the API is not finalized yet and isn't even supposed to be accessible to OAuth applications (like Reviewable), so this may well break without notice. I'm also not surfacing any annotations attached to a check at this point; I don't know of any clients that produce those, but if you have a use case please let me know!