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 am not very comfortable with exposing the modelBuilder as a public property. What are your reasons for making it public?

The same applies to `ModelBaseClass`. I don't think it should be exposed at all, IMO it's a private matter of `lib/model.js`.