Custom User Ids for User Restricted Resource Access.

The feature now supports custom User Ids, allowing for more flexibility
and token revocation with token-based authentication.

- 'AUTH_USERNAME_FIELD' renamed to 'AUTH_FIELD'
- 'auth_username_field' renamed to 'auth_field'
- BasicAuth and subclasses now support the user_id property

This change breaks backward compatibility. If you want to preserve the
old 'AUTH_USERNAME_FIELD' behavior, just set 'self.user_id' to
'username'.

Closes #73.