swapped HardwareSerial for Stream in Firmata.cpp

I realized that Firmata.cpp should include Stream.h rather than HardwareSerial.h since any type of Stream may be used with Firmata. I've updated the examples to include HardwareSerial.h before Firmata.h.

This change is going to break pretty much every existing sketch that uses Firmata with Serial but I think it's a worthwhile change going forward.

Thoughts?