Generic case and lid - improved

This improves on davidmpye's generic case (box and lid) from http://www.thingiverse.com/thing:28608. The changes should provide improved dimensioning over a wider range of parameters. The lid should fit the box better, and an option has been provided to add a mounting flange.

As with davidmpye's design, roundCornersCube.scad from http://www.thingiverse.com/thing:8812 is used for the basic box. nophead's polyhole.scad is used for creating the small holes.

The Rev A files provide a