Printrbot Simple Maker's Edition Spool Holder

Based on LucasSaugen awesome original [Printrbot Simple Spool Holder](http://www.thingiverse.com/thing:118559/). Makes some improvements:

- Thinner base that extends longer to offer more support
- Longer spool arm for wider spools, such as MG Chemical's PLA.
- Spool arm is cutout at sides for a quicker print without risking structural integrity.
- Wider spool arm cap to stop spools from popping off.
- Guided slots for cable ties.
- Rounded edges.

Should work with all non-metal (i