Kick off async process on github posts. Add command for easier testing.

This PR contains some controversial elements.

The main two being:
  - the decision to make celery a system wide runner and not just nestled inside the github package.
  - the addition of github as a django ``INSTALLED_APP``

I began by writing a celery app just within the github app, but decided that it made more sense for the github app to define the tasks it needs, but not be responsible for running celery itself.

Additionally, if settings is just seen as the final configuration layer when setting up ployst, and it can contain settings like ``GITHUB_HOOK_TOKEN``, then it makes sense for github to be an app listed in ``INSTALLED_APPS``. (And this had the benefit of making it exceptionally easy to use a centralized celery app).

I think that we should, however, continue to develop against the RESTful API - despite the fact that performance wise it makes more sense for the client to be using django models directly. The reason being it will force us to make the API robust and this will certainly be useful when extending the angular client.