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
The registry should be responsible for interacting with loopback-datasource-juggler, not the base Model class.

I don't like exposing ModelBaseClass too much, but it's needed for this code to work:

var Model = module.exports = registry.modelBuilder.define('Model', {}, {}, registry.ModelBaseClass);

The idea is that even the default base model class is set to Model, but Model itself should be inherited from ModelBaseClass.