Updated linked accounts to work with Django 1.5 and custom user models.

User get_user_model() to retrieve user model.
Moved automatic user creation to the separate method in AuthCallback,
so we can override it easily for custom user models.
Removed verify_url from LinkedAccount model, since it is removed in new Django.
Bumped up version in setup.py to 2.2.
Allow to pass Google Specific 'hd' and 'user_id' variables
in the login url, which allows as to hint on authentication only
with certain Google Apps domain/email.