Move phpcr mappings out of default location to avoid conflicts with mongdb/allow auto_mapping

Fixes #509

I think its possible users could see the same conflict when they run easy extends because it would create application classes in the Document namespace for mongo but these would not be needed (the user can delete these if they are using PHPCR) and therefore easily circumvent the problem, although whether it would be clear to users that they needed to do this is unlikely (they are going to get a generic error saying the Application/MediaBundle/Document/Gallery has invalid mappings). On the other hand there is no reason to commit Document classes to your repo that you arent even using so if users are paying attention they can avoid the issue. Still seems problematic, perhaps something for the easy extends documentation? Or the media bundle docs?