Adding support for UNL2003 Stepper Driver

This adds support for cheap stepper motors with UNL2003 drivers ( less than $5 US: <a href="http://www.dx.com/p/5v-stepper-motor-uln2003-driver-board-set-158635?Utm_rid=94442630&Utm_source=affiliate">DealExtreme</a> ).  I've added comments that give a good start for tuning, but it's not perfect.  Also, I've added in a wiring diagram to get the drivers hooked up.  (The code is probably not the best place to put the diagram and instructions, but oh well)