Add support for the combined view of commit statuses

Add the combined status support,
- https://developer.github.com/changes/2014-07-09-status-contexts-are-official/
- https://developer.github.com/v3/repos/statuses/#get-the-combined-status-for-a-specific-ref

And replace the legacy route of https://developer.github.com/v3/repos/statuses/#list-statuses-for-a-specific-ref by the new one