Eustathios Belt Tensioner

Remix of Eric Lien's [Eustathios v2 Spyder](https://github.com/eclsnowman/Eustathios-Spider-V2) belt tensioner. This version is designed to work with 10mm cross bars, 30mm graphite bushings (still uses a 15.9mm diameter opening), and allows the cross rods to be removed without a full disassembly of the x-y gantry. (update: it turns out that the cross bars can be removed from the original belt tensioners by sliding the bar to one side and lifting it out)

A version for 8mm cross bars is inc