Itopie X-carriage (3 bearings) with aluminium 20 pulley

X-carriage for itopie, with 3 bearings, but its possible to in generic prusa I3.
No need zip tie and its simetric to use with aluminium 20 pulley.

In itopie, i think it's really a problem to use 4, it creates more friction and consequently makes the said resonance, mainly because I use igus bushes.

I also changed the parts of the x axis, to tension the rods and to use a lead screw.
The parts are these: http://www.thingiverse.com/thing:1315535

With this change I reduced the resonance p