Modified Greg's X-Carriage with double fan mount

This is based on Greg's x-carriage with fan mount modified by binaryconstruct.

I've made a few modifications:

• Removed 1 linear bearing, leaving only 3 as the new It2 Prusa;
• Added another fan mount on the other side of the x-carriage;
• Minor adjustments on the model (removed unused holes, etc);
• Adjusted the clamps for my GT2 2mm belt;