Need support to get "combined status for a specific ref"

[prabinb]

Please refer to this githug api - https://developer.github.com/v3/repos/statuses/#get-the-combined-status-for-a-specific-ref

Currently there in the library there is no good way to find the combined status of a ref. I believe support for this end point would be a good use case.

[bitwiseman]

This will involve creating a new class GHStatus to then adding methods to GHRef and/or GHRepository which return this. Not the best first issue, but still pretty straightforward.