Alex/280 github provider

[alexcouper]

This almost completes the github provider flow. At the very least it shows the way it is going to happen.

The test found in test_end_to_end.py, which mocks out core responses shows it "working". However there are a few missing pieces before it can be used in reality:

• [x] Provider-specific settings. This concept is missing, as is an api call.
• [x] Updating branch information. This API call is missing.

Once these are done, the github provider should be working.

However, until we have the following, it won't be particularly useful:

• Ability to select the repos to monitor
• A public facing address to which github can POST

Additionally, I think we should do the following clean up after we have got this working fully:

• [ ] Make the recalculate method a background running task to be taken care of after responding to github.
• [x] Tidy up the client to be a class with a url (and token) passed in to the constructor.

[txels]

Looks like a nice substantial piece of work. I'll properly review this within the next days.

[alexcouper]

I think I've dealt with all comments.

If not, let me know and I'll come back in to this.

Once this has been merged I'll look at asyncing the github hook process.

[txels]

It all looks good. I'm trying to make the tests run, but there is one simple failure I can fix myself and merge