Add Tower of Hanoi demo

Adds a Tower of Hanoi game which uses the Gertboard's switches as a
primary controller by interfacing with the GPIO. Originating as an
extension of the buttons demo, this program aims to give an example of
somewhat more advanced use of the Gertboard and the C programming
language.
toh: Clear the screen between moves
Force remove files in make clean target

The rm command will fail if it attempts to return a non-existent
file. To prevent make clean from failing unnecessarily, rm should be
forced.