support bundle alias notation

Implement https://github.com/symfony/symfony/pull/10853 so that `FOSUserBundle:User` works again.

I guess this is a rare case, as people will extend the user class and then have doctrine do the alias for their extending class. But for consistency, it makes sense to do this.
if we have symfony 2.6 but DoctrineBundle < 1.3 the alias support will still not work. but i don't think its worth starting to introspect the orm compiler pass and whatnot.

or we could simply never use the compiler pass from symfony but always our local compiler pass...