Printrbot base with z-motor support.

The standard plastic printrbot base allows the z-motor to rock - this one doesn't.

Optionally you can also use linear bearing clamps instead of zip-ties to hold the y-bearings in place.
My tested clamps have 4 holes for screws. I've added other two holes to this version. This is not because you need 6 but because you have the choice to use fewer - 2 may be enough but I haven't tested this.
The zip-tie slots are still there if you want them.

Also the motor has been moved 0.5mm to match th