WIP: [feature/auth refactoring] Authentication Plugin Refactoring and User Integration

RFC: http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=42895
Ticket: http://tracker.phpbb.com/browse/PHPBB3-9734

Refactoring of existing authentication code to separate it out into gui and backend code instead of leaving it integrated. Add support for logging in with and registering with third party services such as Facebook and OpenID.