[v2.0] Prusa i3 X-axis lead screw upgrade

Next stage of our printer's "evolution." Fully redesigned the leadscrew upgrade off the original Prusa frame onto a more sturdy and spacious 15mm aluminum extrusion frame, 600mm³, fully enclosed with doors (polycarbonate enclosure not in photos).

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We begin this Thing post with a working X-Carriage:

- *Compatible with our previous Prusa-frame based X-axis lead screw upgrade design* [published here](http://www.thingiverse.com/thing:641033).
- Designed with a common 24mm CL-to-C