Improved lenscap

Based on leemorton123's parametric lens cap but heavily reworked to avoid bridging and overhangs, to give a very functional lens cap. STL files are generated with diameter=76mm in order to fit a 77mm lens.

Prints in 4 parts and clips together.

Updated to address some issues with hole placement on smaller caps

There is a newer version at http://www.thingiverse.com/thing:619943 that supports the online customiser and creates all the parts in a single stl file. Use that in preference to th