Fix Turtle::turn_towards()

[sunjay]

Turtle::turn_towards() (docs, source) is currently very broken and unstable. There is a followmouse example that you can try to see all the problems. It may be that the math in turn_towards is incorrect, or it may be that the example code is buggy. We need someone to run the example and debug what is going on. Once this is complete, the note about the method being unstable should be removed and any tests necessary should be added to the repo.