Scalar - Upgrade Z Axis trapezoïdal screw

# Description
This thing is a mechanical upgrade for Scalar XL 3D printers.
This upgrade allows to use a Tr8x1.5 trapezoïdal leadscrew instead of a standard M8 leadscred on Z Axis.

For the convertion, as the Trapezïdal leadcrew often requires a special nut, the Leadscrew has to be moved a little bit, for the nut to fit the overall design of the Scalar XL.

# Updates:
* 2016.07.28 - Added Nut holder for T8 nuts
* 2016.07.15 - Both tops and Bottom holders have been upgraded
* 2016.07.15