Introduce a commit status API

Reports:

• feedback issue http://feedback.gitlab.com/forums/176466-general/suggestions/3923065-status-api-for-continuous-integration
• Status in commit index http://feedback.gitlab.com/forums/176466-general/suggestions/5608385-show-build-status-in-commit-index
• One major customer in customer electronics asked for this

We should allow external programs to submit a status of a commit. This should be used for GitLab CI and other external testing tools.

1. It should allow multiple programs to submit a status
2. It should allow the overwriting of a previous status (pending => failed => green)
3. States are: pending, success, error, or failure
4. It should be shown everywhere the commit is shown (MR, commit show, commit index, branch overview)
5. GitLab CI should use the same interface (no more getting a picture)

See GitHub post https://github.com/blog/1227-commit-status-api and docs https://developer.github.com/v3/repos/statuses/

And Stash post https://developer.atlassian.com/stash/docs/latest/how-tos/updating-build-status-for-commits.html

Third party CI service (CodeShip) are also asking for this.