Prusa I3 Mk2 Dolly Extruder

A revision of the Prusa I3 Mk2 extruder body. Changes include:

1. Moved the filament feed holes 1mm in the +X direction to provide for the slightly larger
MK8 hobbed pully.
2. Moved the Mk2 style Z-axis sensor mount 2.5mm in the +X direction to allow clearance for
its wire.

This body uses the original mounting holes; no necessary modifications to the carriage, cover, or idler.

Thanks to geoffsim for the inspiration to post this mod!