* Make guest login working with new UserService
* Reuse existing guest username
* Adapted tests

switch to oauth2 authentication which allows facebook, twitter, google
etc authentication

This should send a generated random username to the client that is valid
for the current HTTP-Session.

This project is now under GPLv3 license

In local environment the principal must be casted to User object but in
production environment the username is represented as String in principal
object.

With this patch the application will try a cast to User object, if this
fails, it will do a cast to String - which holds the OpenID identifier.