PrintrBot Zero Friction Spool Holder

I've seen [another design](http://www.thingiverse.com/thing:355889) by [emmett](http://www.thingiverse.com/emmett/about) for a horizontal spool holder that fits on to the Z-bars of the PrintrBot but I found that the friction on the spool was a little too great and gave artefacts on the print when the filament was being pulled.

I think that mounting the spool horizontally gives a very neat and stable design so I produced my own one that contains a built-in bearing to minimise friction (oka