Changed ^output-link semantics for motors

The values recognized to control the motors on the ^output-link are now `forward', `reverse', `brake', and `off', instead of an integral value. The integral value didn't make much sense since the motor is either `on' or `off' -- there is now `slow' or `fast'. Also, this allows you to brake (or lock) the motor, as well as turn it off so that it `freewheels'.