Y Tensioner 10 mm - Prusa i3 Rework

Y tensioner 10mm for Prusa i3 Rework and Classic

- Nut retainer on the pulley side to fit a M4 nut.
- Added a slot to fit an internal M3 nut instead of using one on the inside where the rods go.
- Designed to use these bearings / adapter: http://www.thingiverse.com/thing:98787


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