Support for serial

[whaleygeek]

I thought about this, given that to use this board, you need serial anyway, and there seemed no point. However, if you want to use two serial for external sensors, or serial at the same time as GPIO, it's possibly easier to do it all from the one board.

So, there should be a serial abstraction at the Python end that just 'looks like' pyserial, but redirects characters to the appropriate pins on the arduino.