Printrbot Simple Metal Filament Spool Holder

A better spool holder for the Printrbot Simple Metal that is compatible with the aluminum carry handle and any size filament spool. This was inspired by, and is compatible with spool hubs from this free-spinning filament spool holder: http://www.thingiverse.com/thing:462624

Use the [filament guide](http://www.thingiverse.com/thing:1706216) to prevent tangling of the filament in case it unspools too much.
Use the 55mm spool hub to better support larger spools.