Use the combined status API

This branch adds the "jenkins" context to statuses so other tools can update the status of a pull request in separate contexts.

More info:
https://developer.github.com/v3/repos/statuses/#get-the-combined-status-for-a-specific-ref
I think the context should be configurable and defaulted to Appname. 
The current master branch provides configuration jelly in <code>GitHubCommitNotifier</code>