classrobot: Stop on Exceotion

Do the sensible thing and stop the motors when breaking out of the robot
loop()
Also add a stop() function to main.py
tanksteer: Set default speed to 0.5 x max
linetask: Add a bit more intelligence

Slows a wheel to turn instead of speeding up. Add some notion of getting
lost. Needs more sofisticated search algorithm for when it gets *really*
lost.