[DBAL-81] Add auto-commit DBAL configuration option

This makes DBAL auto-commit mode (introduced in [DBAL-81](http://www.doctrine-project.org/jira/browse/DBAL-81) / https://github.com/doctrine/dbal/pull/409) configurable in DoctrineBundle.
Please note that this is my first PR to this repo, so if I missed anything, please shout at me :)