Parametric Shaft coupler with hex support

This is a small enhancement to marcsulf's excellent parametric Z coupler. Our robotics team needed a more general coupler that could support hex shafts and also be non-flexible.

1/26/2014 - Attached is the STL file for a version that prints
on it's side. After testing we found that printing that way was stronger. The insides of the hex would tend to strip before the part delaminated.
Do Not print with PLA - Too brittle
If printing with ABS print with 4-5 shells and high (50%) recti