Prusa i3 Z-axis leadscrew with 4 holes copper nut (add belt tightener)

2015/03/17 : Add belt tightener
This is a design for Prusa i3 z-axis upgrade to leadscrew with 4 holes copper nut.
1. 4 holes copper nut type.
2. Z axis force for nuts is a pushing direction not pulling. More strength.
3. Design with LM8UU bottom stopper. LM8UU won't fall out for long term use.
4. Design with LM8UU top cover (by nut). LM8UU is held tightly inside. (use M3x20 screw to hold LM8UU from the side on the top)
5. Design with LM8UU inner tighten tolerance. Easy to insert LM