Upgrades for Prusa i3 rework

I've been working on some upgrades for my Prusa i3 rework.
Openscad files available for customization (they are a little bit obscure but as I will try to clean them and add comments).

Y axis upgrades:
- Replace LM8UU bearing for 8-12-15 bronze bushing
- Replace idler and fix belt aligning problem (uses M6 screw with nylon nut and 2 MF126ZZ)
- Replace motor holder
Removed end stop holder and reinforced
I've rendered a version to mount on the back and invert the Y axis as in my case the