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
If you create a variable, the annotation won't be true.
After Jenkins startup, all existing Notifiers will have null field. The logic should handle it