Heat Bed Cable Chain for Printrbot Simple Metal

The Printrbot heated bed doesn't have a decent cable management system so here's a cable chain that feeds lines from under the printrbot to the heated bed.

* Remix of chain link from http://www.thingiverse.com/thing:11978 with a flattened bottom for ease of print.
* Chain links remixed with easy-snap connector.
* Heat bed mechanism slides in over the wires and snaps onto heat bed.
* Heat bed mechanism has holes for wire tie to provide strain relief.
* The bed bottom protects the wires