Printable cable chain link

This is based on [thing:1078216](http://www.thingiverse.com/thing:1078216) which was "not optimized for printing". This has been modified fairly extensively to make it easily printable: thicker walls to give it the correct strength, tweaking the gaps to make it sturdy but flexible, slight adjustments to the slopes and orientation so it would not need supports, etc. This version of it prints satisfactorily without supports or rafts on my TAZ: the links are ready to snap together as soon as they h