RodBot Upgrade - Quaddruple Z-axis rods

After suggestion from [mikehu](http://www.thingiverse.com/mikehu/) I designed a small upgrade for the Z-axis which some people might find useful. Instead of using two rods on each side of the Z-axis this upgrade uses four with the two extra placed a bit to the side of the first two. This will hopefully make the axis a bit more stable.

I haven't printed these parts yet so I can't say if it will work or not, therefore "work-in-progress".

A 3D pdf is available [here](https://github.com/hampus