Make parent controller configurable

As we discussed, I'd like the parent controller to be configurable so that I can, for instance, use my AdminController as my parent controller. I've made a suggestion.

I'm not entirely happy with the change in the NetzkeController. I wanted to add a @@parent_controller_class declaration to Netzke::Core, but Netzke::Core doesn't seem to know about the ApplicationController class. So this is my suggestion.

Tests pass. I considered adding a ParentController inheriting from ApplicationController to the test app and configuring that in the netzke initializer, but that felt like overdoing it.

Cheers!