Reverse Bowden for Prusa i3

Here's a reverse-Bowden setup for the Prusa I3 MK2 and MK3. It's called "reverse-Bowden" because the extruder doesn't push filament through a PTFE tube into the hot-end. Instead, it pulls the filament through the tube. There are a few advantages (real or imagined) to using a reverse-Bowden setup for the Prusa i3. It allows you to take the heavy filament spool off the frame. It eliminates the need for a filament guide. Also, travel movements of the print head don't cause the filament to un-spool (a real problem with PETG.) In my design, retraction is not affected. The PTFE tube sits, unattached, on top of the extruder and lifts up on retractions instead of having to push the filament back up the tube. (May not be a real problem, but it's one less thing to worry about.) Note that I have, once again, taken Area51's clip design (see http://www.thingiverse.com/thing:1940105) and used it in my own design. I have reverse-engineered it into OpenSCAD to make it easier to fit p