Simplify Commander API

[runger1101001]

I think by introducing a "command type" we can get rid of the need to make callbacks for Commander. We could still keep a "callback command type" so callbacks are still possible (since they are useful) but for all the standard types supported by Commander like motor, pid, scalar we could get rid of those callbacks and just keep an internal table of command letters to command types, and call the commander.motor() function directly.

So the user calls just addMotor('M', &motor); and Commander does the rest internally.