iamburny's extruder gears with adjustable hole sizes

iamburny's extruder gears are a great match for the spring-loaded Ormerod extruder from masonstonehenge, see http://www.thingiverse.com/thing:318458. However, after using slic3r to generate gcode and then printing them, I invariably found that the holes for the hex bolt head and the stepper motor shaft were too small. So I have made them parametric in this remix. The parameters at the top of the OpenScad file are just right for my 3D printer, but you may need to adjust them for yours.