Prusa i3 - Semi-Utilitronic Industries edition

Plastic parts for the Prusa i3 kit I sell (http://www.emakershop.com/Seller=801).

Edits to the original (https://github.com/josefprusa/Prusa3, https://github.com/josefprusa/Prusa3-vanilla, extruder: http://www.thingiverse.com/thing:18379):
- minimal Prusa i3 x ends: zip ties, built-in adjustable tensioner and easy z height adjustor (http://www.thingiverse.com/thing:500550)
- increased height on y corners to avoid clearance issue with bed mountings.
- combined mounting holes for hot end