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..)~~
[here](https://github.com/doctrine/doctrine2/blob/e7be0c4dc98715ab1154a11bafd1a3967ce6970c/lib/Doctrine/ORM/UnitOfWork.php#L1830) the UoW tries to load the entity, and `$flatId` comes from identifier flattener, that should load the required metadata.

This problem is present even with the previous implementation (see `getIndividualValue`...), and can be treated in a different PR.