[WIP] Context Awareness: Implement commit status contexts

Back in March, Github released the [Combined status API](https://developer.github.com/changes/2014-03-27-combined-status-api/). As it nears the end of its developer preview period, the organization I work for has found need to start leveraging them. From what I can tell, this is working as intended. But GitHub doesn't seem to be reflecting these in their UI yet. :cry:

That said, I think this could still be useful for teams that want it so I decided to go ahead and open the pull request. This PR can't be merged until release 1.55 of the github-api jenkins plugin is out the door. The java dependency itself has been released, but it looks like when they tried to release the plugin things didn't go as planned? I've emailed the author about it, and am still waiting on a response.

So, things we need:

- [ ] github-api-plugin 1.55 being released.
- [ ] Another pass through the tests, because I'm about 99% sure my brain isn't functioning correctly today. heh.

closes #155