MakerFarm i3v X or Y idler belt pulley for 12mm bearings

This is a remix of Zen1's i3v push fit belt guide from http://www.thingiverse.com/thing:359773, leveraging an enhanced version of the original source openSCAD script from Parametric Pulley, http://www.thingiverse.com/thing:16627. The pulley can be used for either the x-axis or y-axis idler in the MakerFarm i3v printer.

Changes to the thing:359773 design include increasing the pulley_t_ht parameter from 6.2 to 6.5mm to provide more room for the belt fit; decreasing the idler_ht and retainer_ht