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..)~~
@goetas the identifier fields can be checked upfront, so that you know whether to expect an association (required to be mapped) or a scalar (which may also be, for example, a `DateTime` instance)