Example OpenSCAD Screw Thread Transformation

Inspired by [speedymollusc's openscad screw threads](http://www.thingiverse.com/thing:368073), this is an example function for transforming a "radial" polygon representation of screw threads into an "axial" polygon suitable for linear_extrude with rotation.

By "radial" representation, I mean a list of coordinates [[ r1, z1 ], [ r2, z2 ], ... ] where each pair contains a radius and z position (dimension parallel to axis of screw). Such a polygon will generate a thread-like shape when used wit