Added a way to enable auto_mapping option using multipe document managers

This PR adds to DoctrineMongoDBBundle the same functionality of https://github.com/doctrine/DoctrineBundle/issues/60 

It requires the approval of https://github.com/symfony/symfony/pull/11815 and would require a new min-version constraint for `symfony/doctrine-bridge` (eg: `~2.6` if merged in SF 2.6...).
you can simplify this as you don't use the ``$vendor`` argument