Fixed the removal of event listeners

Making the listener abstract so that it is removed without being registered does not work anymore because Symfony now registers event listeners earlier than the removal. The bundle was using a hack working because of a bad behavior of FrameworkBundle (which was making debugging hard and so has been fixed)
So it is now registered in the dispatcher and then removed afetr that because it is abstract, thus breaking the mailer.

Please merge it ASAP as the bundle is now broken with the dev versions of Symfony 2.2, 2.3 and master