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.e Maker's Edition) Printrbot Simple models.

V2 Notes:

- Made taller as Printrbot Simple can print a little over 100mm. Extra height will accommodate wider spools.
- Added lower zip tie holders to stop the piece flipping upward. Zip ties will wrapping around back of the Printrboard.
- Filleted the end to reduce overhang errors.