X-Axis Tensioner Arm for toothed Idler Pulley

This is meant to be an addition to the [Smooth X-axis for Prusa i3 by MazaaFIN](http://www.thingiverse.com/thing:1103976) as I wanted to use a toothed idler pulley instead of two small flanged ball bearings. Unfortunately, the tensioner arms he designed do not accommodate the increased width of the toothed pulley.

So I designed a new arm from scratch based on the basic measurement that would fit in the tensioner bracket. It is intended for idlers with 16 teeth (14mm diameter, 8.5mm with, M3 a