Timing Belt X MXL, Printrbot Simple V2

**This will work for both V2 designs (with & w/o string tension)**

After successfully adding the timing belt to the Y axis, it was time to convert the X axis. Assembly method is similar to my Y axis conversion. http://www.thingiverse.com/thing:180146

REV B:
- Add alternate X Belt End Mirror for Simple V2 With string tension device (models ordered after 10-23-2013).
- Add chamfer to belt ends to give a little more clearance for motor plate at both ends of travel.