
- make Firmata a subclass of HardwareSerial

- per-pin digital callback, since the per-port callback is a bit complicated
  for beginners (maybe Firmata is not for beginners...)

- simplify SimpleDigitalFirmata, take out the code that checks to see if the
  data has changed, since it is a bit complicated for this example.  Ideally
  this example would be based on a call

- turn current SimpleDigitalFirmata into DigitalPortFirmata for a more complex
  example using the code which checks for changes before doing anything

- test integration with Wiring
