Ultibots Kossel V-slot magball roller carriage with tension adjust

###Note 12/22/2017: Updated D version has some minor reinforcements and some other small refinements. Created it after a failure on a PLA printed carriage of mine. Probably wouldn't of had any problems if I had used ABS or PETG, but hey, figured it was worth some modification.

This is a tension-adjustable V-Slot magball carriage (42mm spacing) derived from the Ultibots experimental carriage for the [D300VS](http://www.ultibots.com/) by Brad Norman found [here](https://github.com/UltiBots