Dumbbell weight plate bushing

A couple of my dumbbell bars came with rubber washers, which have gotten hard and broken. This is to replace them: it's designed to fit relatively tightly on the dumbbell threads and into the hole of the weight plate, so that there's no play when it's tightened down.

Based on screw threads from [Patola's star nut](http://www.thingiverse.com/thing:1239537 "thing:1239537"), and as with that design, depends on [aubenc's polyScrewThread library](http://www.thingiverse.com/thing:8796), which is duplicated here. (It works well enough to have it in the same directory, or you can install it in one of the [OpenSCAD library locations](https://en.wikibooks.org/wiki/OpenSCAD_User_Manual/Libraries#Library_Locations).)