with a BytesFileSystemStorage POC implementation.

Basically:

* a dictionary contains maps from which attribute of which entity types are
  mapped to which custom storage

* hooks check for one of these entity type being added/modified/deleted

* read is based on the sql generator callback mecanism (used in vcsfile for
  instance)

* all storages have the same basic interface (read, add, update, delete),
  and should be pluggable in a transparent way (except at migration time
  when one want to change from a storage to another)

* the sample BytesFileSystemStorage:
  * may store Bytes attributes content of any entity type as file on the file system
  * is based on one FSPATH rql/sql function and another _fsopen only available in sql
  * has a dumb file name allocation algorithm