Sparkfun Monster Motor Controller test app

Takes simple commands over serial port to control 2 channels on the
Sparkfun Monster Motor Controller.

Example command:
F50.0 R25.0 ; Right motor 'F'orward at 50% duty cycle and
            ; left motor 'R'everse at 25% duty cycle.
-0 +0 ; Both motors are braking.  Right motor brakes to -0V and
      ; left motor brakes to +5V.
F100 F100 10000 ; Both motors run forward at 100% duty cycle and
                ; set PWM frequency to 10kHz.  Starts up at 20 kHz
                ; and only changes when this 3rd param is specified.

A blank line or other invalid input will stop both motors.