Customizable omni wheel prototype

*THIS THING IS NOT YET FINISHED OR TESTED*

This is an update to [my original omni wheel](http://www.thingiverse.com/thing:788274), this time parameterized in OpenSCAD, which I just learned. I still haven't gotten the O-rings in yet, so I've only been able to test basic fitting of the pieces, not if it actually works as an omni wheel.

The project is to remix [Arcwave's O-ring based omni wheel](http://www.thingiverse.com/thing:515831) with [mbbackus's split-ring one](http://www.thingiverse.com/thing:395085).

Basically, Arcwave's O-ring idea is great for traction, but the spinners are on plastic axles in a plastic housing, so the rolling friction seems bad.

mbbackus's use of a split ring to provide the axle is awesome, but the spinners he uses are washers, so it seems like traction would suffer.

Lastly, I don't need them to be as big as either design, and I had 1.5" split rings on hand, so mine are also tiny, and use 8mm ID O-rings instead of 1/2" ones.

I haven't gotte