Add 2MHz and 4MHz sample rate support.

Use unrolled loops to sample at 2MHz & 4MHz rates.  Based on some
testing by Bob Davis (http://bobdavis321.blogspot.com)
The maximum with a 16MHz clock is 5.3333MHz (3 cycles per sample) but
sampling at that rate isn't very accurate.  Accuracy is pretty good at
2MHz & 4MHz.