Printrbot jr. z axis improvement

Replacement bracket for the z axis nut holder in a Printrbot Jr.
Reduces the amount of play the z axis nut had to a minimum.
Replaces the original threaded rod and nut with stainless steel M8 threaded rod,
two nuts and a spring in between the two nuts. Ideally you should use brass nuts.
But I used stainless steel nuts since they were available and work just fine.