Add Github Commit Status status plugin.

Github recently added new commit status API. This status plugin adds support for this new API to the buildbot.

Example usage:

```python
from buildbot.status import status_github_commit
c['status'].append(status_github_commit.StatusGithubCommit(username='username', password='password', account='account / organization', repository='repository name'))
```

![](http://img6.imagebanana.com/img/6yv48tuq/Selection_055.png)

![](http://img6.imagebanana.com/img/hzf042zs/Selection_056.png)
* https://github.com/blog/1227-commit-status-api
* http://developer.github.com/v3/repos/statuses/