Prusa Z Motor Mount

Derived from http://www.thingiverse.com/thing:12151, which I though was a really good idea, just wasn't sure about tightening the outer nut without a nut the other side of the relatively thin bit of plastic, so I extended it for the entire length. Also made the vertex clamp area thicker. Downside is it uses more plastic than the source, and you can't use large washers as they won't fit in the vertex area.