Z-shaft standoff

This is an "add-on" for [andreasL's alternate Z-axis](http://www.thingiverse.com/thing:896556) for the SmartRap Core. I like my bed to be well supported when at it's lowest position for cleaning so I designed these simple stand-offs that clip on to the 8mm shafts in my printer. They support the bed on both sides quite well.

The [OpenJSCAD](http://openjscad.org/) code is straight-forward:

function main(){
return difference(
union(
cylinder({r: 10, h: 5}),
cylinder(