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 more refined utilization of openSCAD for the designs. More dimensions can be individually tailored. For example, the wall thickness, base thickness, and lid thickness can all be set independently. A table of reference dimension data for common hardware is included in the source file. The mesh overlap between elements has been improved to minimize "ghosting" in the render view. Echo outputs have been added to the console output for key calculation results. The path to roundCornersCube and pol