Upgraded Greg's Extruder for Makerbot

I have changed one major thing and several minor things on the original design.

Major change - The original gear on the Makerbot meant for gripping the plastic can be used on this extruder. This translates to superb grip compared to using only the screw's threads to grip the plastic.

Minor changes:
-Flipped the mount, so Stepper motor doesn't run out of cable length.
-Extruder holes moved, mounts DIRECTLY onto the Makerbot.
-Small gear center hole larger to fit on Makerbot's Stepstruder