[WIP] model + adapters

Related to:
- https://github.com/symfony-cmf/cmf-sandbox/pull/182
- https://github.com/symfony-cmf/BlockBundle/pull/72
- https://github.com/doctrine/phpcr-odm/pull/293
- https://github.com/doctrine/DoctrinePHPCRBundle/pull/72
- https://github.com/symfony-cmf/CreateBundle/pull/60