Enhancement Request: active braking for PwmMotor

[sparry0407]

Currently PwmMotor implements a coasting stop: when you set the speed to zero or stop it sets both the forward and backward channels of the bridge to zero, allowing the motor to continue freely rotating. Other implementations seem to include an active braking approach, allowing you to set both forward and backward channels to 1.0, which causes the wheels to stop dead and also helps keep the vehicle from rolling away. Could we please have a brake function? I have implemented a variation of PwmMotor for my own use, which appears to work well. The only hitch I found was that I had to add forward and backward stop calls to the close function to stop random wheel movements during close down.

[mattjlewis]

Will take a look