GH-232: Recursive check for entity identifiers and hashes

Pull-Request was automatically synchronized: https://github.com/doctrine/doctrine2/pull/232

Hi all!
This PR will add a better support for entities with association keys.

getType will check recursively to find a type for the identifier.
getIndividualValue will search recursively to find the identifier value

trygetById improved, using a recursive function to find an id value instead of implode functions (that cause exceptions if the identifier is an object and do not implements __toString method).