Revamping extension system

It's now possible (and encouraged) to do:

  var MyModel = Composer.Model.extend({ ... });
  var MyModelBetter = MyModel.extend({ ... });

Before, the process was uglier:

  var MyModel = Composer.Model.extend({ ... });
  var MyModelBetter = Composer.Model.extend({ ... }, MyModel);

This has been tested/updated in all exported classes and is working
nicely.