`DocumentManager#transactional()` support

See [`Doctrine\ORM\EntityManagerInterface`](https://github.com/doctrine/doctrine2/blob/5a6bbbd6241d803b686a13a0c373e0febe656987/lib/Doctrine/ORM/EntityManagerInterface.php#L70-L84).

This is required for doctrine/data-fixtures#164
Handled (diff is on separate lines)