use the new doctrine compiler pass to map the model classes directly

this is to show how we would use the compiler pass of the doctrine bundle to map our model classes directly. See https://github.com/symfony/symfony/pull/7599

this is done for orm, mongo and couch. we have a simplified compiler pass in this bundle for backwards compatibility with symfony < 2.3. that can be removed once this bundle is declared to only work with symfony 2.3.