Added increment and decrement functions

Hi there,
   I've added a couple of extra functions to your library, to include the single increment and decrement commands. 

I needed them to bypass the full speed timeout feature built into the lego power function receivers. I used your library in the making the control of a monorail mounted video camera for an recent instillation.

more information, pictures and video links can be found here.
https://github.com/paulhayes/DarkRide