NEMA 17 bowden extruder drive

Based on Wade's extruder, with the X carriage mounting holes removed since this will be mounted on the Huxley frame instead. Also includes some other changes to take the bowden tube (3mm OD) and work with a 1.75mm diameter filament, driven by an M4 toothed brass insert.

The gears are 9 and 53 tooth gears respectively, giving a ratio of nearly 5.9:1. I might try using a NEMA14 instead in future since the force required to push 1.75mm filament through the nozzle is a lot less than 3mm filament.

The huxley_gears.scad file uses Greg Frost's involute gear script http://www.thingiverse.com/thing:3575

I'm not totally happy with how this mounts onto the frame yet, but it's useable (with a few zip ties) and running on my eMAKER Huxley prototype machine.