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.