Leave the definition of models to the bootstrapper

Depends on strongloop/loopback-boot#11.

/to @ritch 
Why can't we just `require('loopback')`?

I could see one of these reasons, but I'm not convinced they are valid yet:
 - you aren't gaurenteed to get the right instance of loopback, unless you constantly run `npm dedup`.
 - all dependencies should be passed as arguments (not sure I agree with this...)