Refactor modelBuilder to registry and set up default model

/to @bajtos 
/cc @ritch 

The PR sets up the base model class to be Model for data sources created by loopback.

See https://groups.google.com/forum/#!topic/loopbackjs/kaiUJ6zLmBA
I'm not sure if it's a good idea to registry.PersistedModel. We have connectors that don't support CRUD at all.