Remove unused code from gauth/__init__.py which had been copy/pasted from django.contrib.auth.

Tickle #49.
Changing the `GaeAbstractUser` back to having 'username` instead of 'user_id' as its USERNAME_FIELD.

This means that users can be authenticated with either the Google Accounts API or the standard Django auth.

Also gets rid of the need for a custom user manager.

Touch: #49.
Adding functionality to allow User objects for Google-Accounts-based users to be pre-created before they log in.

Needs tests.

Touch #49.