Prusa I3 spool holder arm

UPDATE 14/07/2015:

- Made de nut hole a bit tighter
- Added a lug on top of the frame holder.
That's why there is now a right and a left version for each arm.

FIRST VERSION:

New shape and slightly less thickness to use less plastic.

Uses standard M4x20 screw with nut hole to tighten to the frame.

Versions for :
- 5, 6 and 8mm frame thickness
- 6 and 8 mm threaded shaft diameter.