RepRapPro Huxley Pen Holder

This is my huxlyified version of geoffd123's pen holder.
The changes I made were to move the mounting points to the correct places for he huxleys x-carriage, change the elastic band fittings to a simple hook rather than a 3mm bolt and change the design to use square nuts (I find these behave better than the regular ones when trapped inside the part).