added friction parameter

Set to a small floating point number for natural-feeling "air friction".
With gravity, a "terminal velocity" naturally emerges.

I tried to make the change as computationally minimal as possible -- just two subtractions and two multiplies per frame.  I also made the default value of friction 0, which would have no effect, and didn't add it to the XML parsing so that PEX format compatibility is retained.  I understand if you don't want to pull though :)