PHPCR add missing features + document LiipImaginebundle integration

This PR adds the following features for PHPCR:

* pixlr (fixed urls etc., not tested it in production)
* gallery
* LiipImagineBundle integration

Furthermore:

* verified this update in an ORM project to validate nothing breaks
* filters for PHPCR are commented because currently no service is attached to the filter types: string, checkbox. As soon as this is solved in SonataDoctrinePhpcrAdminBundle it can be uncommented