Leave the definition of models to the bootstrapper

Depends on strongloop/loopback-boot#11.

/to @ritch 
What I wanted to say - we should not add wrappers for the sake of model js files. We should stop and re-consider where is the best place for the function to live. Only after we came to the conclusion that is should be defined on an object not accessible from Model/Base, it is the time to figure out how to provide that object to the model customisation function.