Update Directional Motor object to support shields that require two pins to specify direction

Some Arduino motor shields (such as the Freetronics H-Bridge Motor Driver Shield (http://www.freetronics.com/products/hbridge-dual-channel-h-bridge-motor-driver-shield)) require two pins to specify the motor direction. These changes allow us to specify that additional pin (called cdir, for counter-direction) in the directional motor object.

There are hardware ways around this, such as connecting the the HBridge's cdir to the dir pin via a NOT gate (as opposed to doing it through the cdir pin being defined here). However, if you are not short on pins, it may be easier to do this in code.

I have updated the docs and inline comments. There was no need to update the schematic, since it already represents a generic H-Bridge.