3FPD-Replicator 2X - Extruder Upgrade

This is an extruder upgrade designed specifically for the Replicator 2X similar to whpthomas, and others, but with improvements.

*** update *** Rev 3 http://www.thingiverse.com/thing:111711

-base is fully printable without supports
-arm is printable with only external supports (bearing slot)
-arm integrates feed tube mount as well
-tension spring geometry improved
-use 5/32" and 1/4" bits for tight clearance bearing/arm pivot holes
-uses Makerbot hardware kit for Rep2 + 1 extrude