Added py2 to travis, and updated README
added disable_turtle ctx manager
fake 2.7 Tkinter too
stug mainloop
stub mainloop properly
Refactor TurtleWorld/PowerTurtle

 - no world parameter passed to callback
 - factored out RawTurtle specific __init__ stuff better
 - added _do_callback wrapper function for better subclassing
 - more docstrings
make random_walk look more like turgles demo