shiftStepper speed boost

I just uploaded a revised version of the shiftStepper library to the Libraries directory, The changes are all speed-related -- I decreased the time required to execute shiftChain::doTick() for a single 16 channel shield with four stepper motors from 231 microseconds to 145 microseconds. In particular, I reduced the portion involved with shifting out the bits from 100 microseconds to 24 microseconds. This should keep the overhead for long chains down.

shiftStepper Arduino Library Intro & Tutorial

I've been working on an Arduino library to drive unipolar steppers with the sixteen channel high current shield. You could do that before, of course, but this will make it much easier.

The major motivation for this, other than simply making my boards more useful, is the possibility of radically cheaper RepRap electronics. One sixteen channel high current shield + one Arduino + one Proto Shield to land the end stops, the thermistor circuit, and a MOSFET to drive the heater should bring the electronics cost down from around $250 to around $100. 

Since I plan other shift register boards besides the six channel and sixteen channel high current boards in the future, this will have to expand to cover various different types of boards doing various types of things on a single daisy chain. I've been holding off on the stepper driver while I figure out how to architect that system in a reasonable way, and I think I've got a workable architecture. Here's a quick video of a sixteen channel high current shield driving four Lin Engineering NEMA 17 unipolar stepper motors.