Switch to 6 channels on PORTD.

Switch from PORTB to PORTD so that a full 6 channels can be used
without messing with the LED.  Per suggestion in issue #7.  I was
unable to find any issues with using PORTB.  During initial development
I ran into some noise & stability issues but I believe those were
solved later via allowing the ports to settle prior to beginning
sampling.
This allows for 6 channels, starting with digital pin 2 (next to the
UART TX pin) and ending at digital pin 7.
The debug pin is now digital pin 8.