Prusa I3 Hephestos improved Z axis brackets

**UPDATE:** After some time using this piece, I've realized that it produces some z-wobble effect in my prints and I've removed from my printer. I have better results freeing the threaded rod. So please, use with caution.

This thing is based on the part made by [@Oneira3D](http://www.thingiverse.com/Oneira3D/about). When I first made the original part, the ball bearing went down through the hole and fell down. I have just added a stop for the ball bearings.

All credits go to original thin