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
Since `modelBuilder.defaultModelBaseClass` changed only after `Model` is defined, we don't need `ModelBaseClass` at all.