PrintrBot Simple Y-Axis GT2 Belt & Extension

Increase the PrintrBot Simple y-axis travel to 160mm AND eliminate string slipping problems by replacing most of the y-axis parts and converting to GT2 belt drive. Twisting and dimensional stiffness better than wood parts even with the 60% increase in travel. Designed to work without modifying any of the original Simple parts so modification can be easily reversed if needed.

Requires 250x100x75 print volume which I was able to do on my simple using [thing 215294](http://www.thingiverse.com/t