Micro-Adjustable Prusa Mendel Z-endstop

While getting my z height correct and playing around with different bed materials, I found myself constantly adjusting the z endstop. If I tried to leave it loose enough to move, it didn't stay in place, and if it was too tight I couldn't move it.

I needed to be able to adjust it very small amounts to get the z-height just right to get a nice first layer that wasn't too hard to peel off, but still stuck.

This minor improvement does the trick.