added execution interrupt function, faster turtle

Use the stop button to abort execution and reset environment. New
primitives 'fast' and 'slow' change draw modes from a 30 frames per
second rendering speed to almost instantaneous line drawing. Holding
shift sets the frame rate to 60 for a slightly faster but still
perceptible line draw speed.
Merge remote-tracking branch 'origin/master'

Conflicts:
	.gitignore