Ultimate Greg's Wades Geared Extruder -- Bowden M4 nut version, with more constraints for flexible filaments

This is a remix of AndrewBCN's version of the Ultimate Greg's Wades Geared Extruder, with the following mods:
- better constraints for the filament feed hole to aid printing of flexible filaments (I still have issues with FlexiFil though, so your mileage may vary).
- inline-removable (can be dismantled without a full retraction) M4 nut for 2mm/4mm Bowden tubes. Use [this threading tool](http://www.thingiverse.com/thing:892397) to get the nut into the tube with a 4mm hole for ensuring proper