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
Note: when you port this patch to loopback 2.x, don't forget to change the default base class to `registry.PersistedModel`.