Improved the way to load metadata for a single class
Improved the way to load metadata for a single class

There is no need to load metadata for all classes in this case.
Replaces #152

OskarStark/DoctrineBundle,chrisguitarguy/DoctrineBundle,fervo/DoctrineBundle,hason/DoctrineBundle,hacfi/DoctrineBundle,Soullivaneuh/DoctrineBundle,skafandri/DoctrineBundle,gencer/DoctrineBundle,javiereguiluz/DoctrineBundle,Soullivaneuh/DoctrineBundle,OskarStark/DoctrineBundle,doctrine/DoctrineBundle,mikeSimonson/DoctrineBundle,skafandri/DoctrineBundle,nicolas-grekas/DoctrineBundle,dunglas/DoctrineBundle,gencer/DoctrineBundle,mikeSimonson/DoctrineBundle,fabpot/DoctrineBundle,chrisguitarguy/DoctrineBundle,hacfi/DoctrineBundle,javiereguiluz/DoctrineBundle,hason/DoctrineBundle,nicolas-grekas/DoctrineBundle,fabpot/DoctrineBundle,fervo/DoctrineBundle