Several bugfixes and enhancements (from several teachers who use Python in secondary and post-secondary classes) to improve usability in the classroom: * docstrings added to methods (Toby Donaldson) * added methods to control speed, window geometry and window title. (Vern Ceder) * added Turtle as alias for Pen - students can now create Turtle objects (Toby Donaldson) * default window now larger and centered (Vern Ceder) * added done() function to start main event loop after drawing (handy when running programs in IDLE) (Vern Ceder/Chris Smith) * fixed bug where filled polygons are lowered (Atanas Radenski) * fixed bug in circle() method to use self._fullcircle / 4.0 instead of 90.0 to determine start (Chris Smith) * removed several redundant assignments (Chris Smith) * added second demo which uses new features (Gregor Lindl)