Simple rack and pinion code for reusing in other projects

Based on previously published code for a rack (http://www.thingiverse.com/thing:6011) and gears (http://www.thingiverse.com/thing:3575) this contains code for both a simple rack and a simple pinion with the necessary ammendments and corrections to make them work nicely together. I've dropped many bells and whistles and simplfied things a lot. Hopefully you'll find this useful. Thanks to the original authors!