Utilize GitHub commit status API

[dahlia]

GitHub provides commit status API, so we can report the start/end of deployment using this. See the API docs as well.

1. pending — until the deployment is done.
2. error — if it failed to deploy.
3. failure — if its automated tests (python setup.py test) failed (Run automated tests #6).
4. pending — until collaborators confirm it (Approve/reject voting #4).
5. failure or success
   • failure — if collaborators rejected it.
   • success — if collaborators approved it.