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
> I don't like exposing ModelBaseClass too much, but it's needed for this code to work:
> ```js
var Model = module.exports = registry.modelBuilder.define('Model', {}, {}, registry.ModelBaseClass);
```

My point is that the code above is executed before the code below:

```js
registry.modelBuilder.defaultModelBaseClass = registry.Model;
```

Thus the following should work equally well:

```js
var Model = module.exports = registry.modelBuilder.define('Model', {}, {});
```

Anyways. If you want to pass the `ModelBaseClass` from registry to Model, then at least make it private (`_ModelBaseClass`).