Composite primary key & association entities as keys improvements

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..)~~
I think that actual implementation will work.

Looking at Uow, `UoW::getIndividualValue()` [here](https://github.com/doctrine/doctrine2/blob/master/lib/Doctrine/ORM/Persisters/BasicEntityPersister.php#L1838) uses the same approach.

`hasMetadataFor` should not return `false`, since the object should always come form the entity manager (can arrive from a previous `findBy*` or form a query) .