Makerfarm Y axis Belt Tensioner Remix

Thanks to TES737 for the original design.

The remix is designed for the OpenBuilds smooth idler pulley (http://openbuildspartstore.com/smooth-idler-pulley-kit/). I've also elongated the slots and increased thickness to prevent deformation under the high belt tensions that I like to run.

Recommend 70% cubic infill as this is a high tension part.

Notes:
- You will need a M5x30mm long bolt and 2 m5 washers in addition to the pulley kit
- Most likely you will need to drill the hole out