K8200 Z Axis Upgrade

This design is a direct derivative of the superb http://www.thingiverse.com/thing:1685126 by Charlies3DT.

I have modified the End Stop mount position so that I can use it without the metal extension on the micro switch and added top mounts with bearings for the top of the lead screws. Plus an additional fixing hole on the motor plates.

I have also added anti-kickback nuts. This does mean there are some superfluous screw holes in the Z Carriage plates. But I only added the nuts as an aftert