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
A variable should be created in the class.
<code>appName</code> is not a good choice, because there's an almost similar global variable.