Make CoffeeScript generators more idiomatic

Previously, the generators did not really evoke the idiomatic approach
to extending existing objects that CoffeeScript provides, using the
class/extends syntax. After testing manually-generated Ember apps
myself with this syntax, I've confirmed that at least as of 1.6, the
class/extends syntax can be substituted as a more idiomatic way
of saying `ObjectName = Ember.ObjectType.extend({ .. })` when declaring new Ember objects.