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
In that case at least CRUD connectors should set the base class to `PersistedModel`. Also note that when you create a model via `loopback.createModel` in loopback 2.x, `PersistentModel` is already a default base class.