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
Please improve tests names to make it clear what is the distinction between them - at the moment, they have the same name.

The empty line L38 is redundant, please remove.