Printrbot Simple Bed Extension and GT2 Belt

This is the cheapest way to extend your x axis. Extends up to 180mm using the original printrbot simple print bed.

The parts were also modified to use GT2 belts based from iamjonlawrence's work. See http://www.thingiverse.com/thing:194686 for more details.

I'm also having issues printing the 608BearingCap608 from http://www.thingiverse.com/thing:194686, so I modified it into two parts to make it easier to print but the parts need to be glued together.