X Endstop holder for Prusa i3

This is a variant of thing:649083.

My modifications raise the endstop about 3mm higher than the original.
Also added material in between the clip arms and added two holes for small zipties if clips don't provide enough grip. This is also for 8mm smooth rod.

REV2: Widened the zip tie holes and gave them a little more meat on the outside edges. Clearance between the smoothrod and the zipties were a little tight, so I also moved them down towards the center of the part by 1 mm.

REV3: Wan