http://en.wikipedia.org/wiki/Stepper_motor
http://arduino.cc/en/Tutorial/MotorKnob
http://arduino.cc/en/Reference/StepperUnipolarCircuit
http://arduino.cc/en/Reference/StepperBipolarCircuit
http://arduino.cc/en/reference/stepper
http://www.tigoe.net/pcomp/code/circuits/motors/stepper-motors/
http://arduino.cc/en/Tutorial/StepperUnipolar
https://learn.adafruit.com/adafruit-arduino-lesson-16-stepper-motors/overview
http://makezine.com/video/driving-big-stepper-motors-with-arduino/