Switched from Digits login to Facebook login

What this does:

Drops the old Digits register/login
Provides support for Facebook login
Drops the X-Auth-Service-Provider header on the register endpoint
The X-Verify-Credentials-Authorization should from now on contain the Facebook access token
Changed some error messages and statuses, not too important for the clients
Requires a display name on registration, since we don't have a separate display name screen anymore

Issue #32

@sander-m @datwelk

Roxot / polly