[WIP] Add PHPCR support via db_driver option, adapter and subscriber.

The basics. This works in terms of add the Vich Uploader annotation to a PHPCR document and having the adapter correctly persist the mapped name to the database and move the file to the right location.

TODO

- [x] Tests
- [x] Update docs
- [x] Add doctrine phpcr odm to suggest in composer.json
- [x] Ensure that the inject on load is working
- [x] Ensure that delete on update is working
- [x] Ensure delete on replace is working

I would really love for this bundle to support multiple db drivers because we have PHPCR documents which are part of the CMS (based on symfony cmf) while many of our other tools use doctrine orm so right now using only one db driver makes it so this bundle is only a partial solution. Honestly i dont really see why it would be problematic to enable multiple drivers, seems like it might just be a config change. If you see any other issues I might not be thinking of let me know. I can try to work up a PR for this feature as well after this adapter is complete.
we should say "MongoDB ODM documents, PHPCR ODM documents or ..." here. ODM could mean any object-document mapper. there is an odm for mongo, one for couch, one for phpcr and there could be others in the future.