Implementation of a Node class, the subclasses of which respond to requests at differ...

Implementation of a Node class, the subclasses of which respond to requests at different paths. Implemented Redirect and File nodes, and changed the existing Page class to be a subclass of Node as well. So, in addition to Template-based Pages, Philo can now serve plain static files as well as redirects to other locations.
Also, removed the requirement on django-mptt, as currently none of its features are needed, and I have yet to figure out how to make it work in multi-table inheritance situations.