Added support for composite primary key on findBy methods and Criteria

Hi! 
I tried to implement the matching of entities that uses composite primary keys...

Any suggestion?

BC changes here https://github.com/doctrine/doctrine2/pull/1113/files#diff-c18001a2ca9928743b1e99854ffad33bL1611 (protected method makred as private and changed return type..)
same type here