Don't merge yet - Feature/club 563

This is the WIP code for Facebook signin.  Facebook authentication basically has 5 steps:

- Request FB Access token in client (ember-facebook)
- Save FB access token for user (ember-facebook)
- Authenticate on server with FB Access token (rest framework and python social auth)
- Send token for validated FB credentials (rest framework and python social auth)
- Set validated token in client headers

This PR has a corresponding PR in Bluebottle. That PR contains the ember-facebook JS library to do client side authentication. This PR includes new requirements, python-social-auth and accompanying settings. There's also an authentication view to login a user with FB credentials. 

What remains to be done to make this all work is to set the headers of the client once the backend authenticated the user with FB credentials. This is the last step in the above explanation.