headMove.js uses avatar motor

JS can now tell the avatar how fast to move by setting its "motorVelocity".  The velocity can be specified in the avatar-frame, camera-frame, or world-frame.  The motor's ability to achieve its target velocity can be adjusted by setting the avatar's "motorTimescale" which is the approximate time it will take the motor to ramp up.

I copied Philip's latest headMove.js into the branch and then modified it to use the new avatar motor for smooth movement.