Update of DualExtrude.cpp for the newer output from MakerWare (2014)

Important: Preheat both nozzles before starting print. Modified GCode doesn't wait for second nozzle to heat.

Minor fixup of http://www.thingiverse.com/thing:24195 to dual extrude G1 commands with A|B feed commands, which seems to come out of the latest MakerWare.
Original from http://www.thingiverse.com/thing:23792
I've also put this up on GitHub to show the diff's of what I modified from Joe's original:
https://github.com/simon-clay/3dprinting-gcode-dualextruder-processor