Timing Belt Y MXL, Printrbot Simple V2

**This is for the Simple V2 WITH String tension ordered AFTER 10/23/2013.
For models w/o string tension see http://www.thingiverse.com/thing:180146
The motor mount and tie bar are identical on both MXL designs.**

Having difficulty keeping consistent traction with the string drive on my Simple Y axis, I set out to design a more positive drive.
Design parameters were:
- Timing belt drive
- "Bolt on" solution, no modification of machine required
- Ability to revert back to string