Herringbone Prusa i3 Reworked Greg's Extruder

The extruder was iteratively modified to address the following issues:

1) The old Greg's big gear does not properly trap the 8mm bolt head. These gears were replaced with the herringbone gear set.

2) The base of the extruder (where it mounts to the x-carriage) was thickened as the large herringbone gear has a larger OD than the Greg's big gear. This larger OD prevented attaching the extruder body to the x-carriage. The thickened base now allows the large herringbone gear to rotate freely.