Added a way to enable auto_mapping option using multipe entity managers

Finally, I fond some time to create this PR. This should close https://github.com/doctrine/DoctrineBundle/issues/60

I've also added  a small test. Feedbacks are welcome!

(if merged, this will require some lines on SF documentation...)
seems like a lot of complex logic, that imho should better be in the abstract base class, which is what I did. then again this solution is backwards compatible with earlier Symfony versions I guess.