Reinforced Z-Axis for Prusa i3 Boxed Frame

These are my modifications for the default z-axis pieces.

The pieces provides a more strengthful attach to the box frame, rather than the default ones with 2 holes (1+1 / front+side).

The pieces have:

* __Z-Motor__ 3+1 holes.
* __Z-Rod has__ 2+1 holes.

__Note:__ These were designed for a board thickness of 12mm. If you use a thinner board, the front outermost hole could end between wood boards.