Add Doctrine storage adapter

[pierredup]

Add a storage adapter for Doctrine.

This should include 2 adapters:

• DoctrineORMAdapter
• DoctrineDbalAdapter

The DBAL and ORM should be separate, so that it can be used regardless of wether you use the full ORM or only the Dbal. This should also work independently from the Symfony integration, although with the Symfony integration, we should auto-configure as much as possible