More universal Reprap Prusa i3 x-carriage

Improvements to the already great design:

• Relocated and added new accessory holes to the bottom
• Improved the mounting extruder positions
• Fixed a minor problem with the belt teeth slot
• Angled the bottom belt blocks to make it fit GT2 belts a bit better

Openscad source is on github, here: https://github.com/blackguest/i3_x_carriage