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..)~~
`$class->identifier[0]` is used almost everywhere in the code base, so i can be confident that starts from `0`