Image Update

Update all output images to use fontname=Sans.  Here's a ghetto gallery of both SVGs and PNGs side-by-side (or above-by-below), plus the usual preview:

* http://dg3.dtrt.org/img/dev/test.html (That file is not included in this pull.)
* http://dg3.dtrt.org/en/developer-guide

Three PNGs we were no longer using have been deleted and the README has been updated.  No dot files were modified.

The newly-generated files look identical to the previously-generated files to me on my computer without Times New Roman installed.

Note: if this pull gets accepted, I'll use the same commands to update the images in the HD protocol update.