Should fix #19 and probably fit with #21. But it is not really recursive, as there's no guarantee that an array with numerical indexed elements are collections, and if they are, if they have the same type of primary key.

• Add a ward against trying to normalize a data that is already a snapshot
• CollectionSnapshot
• Unit Tests
• Use Symfony's PropertyAccess component
• Allow to pass some options into the CollectionSnapshot (like a directive to tell the snapshot which sub-snapshot to use for each values of the collection)