Add support for nesting embeddables

This is a possible approach towards adding support for nesting embeddables (embeddables inside embeddables). I'm not sure whether this implementation is the best solution but I think it is something to start with. It seems to work flawlessly so far but I'm not sure if I missed something, so any feedback is welcome ;)
I don't think so. How would you expect to concatenate here? We don't know the naming strategy to use. Also if you have a look at the tests, it is working as expected IMO. The concatenation is done [here](https://github.com/deeky666/doctrine2/blob/nested-embeddables/lib/Doctrine/ORM/Mapping/ClassMetadataInfo.php#L3220-L3230).
Or am I getting you wrong?