Use fontname=Sans for images

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

Note: this is a re-pull request of #111 with pngs run through optipng and the readme updated to suggest optipng.  (I tried to do something without reading the manual and managed to break the previous pull request.)

(If you previously viewed the preview above, you'll probably want to Ctrl-Shift-r refresh to get the new PNG files.)

@saivann if everything looks good to you, I suggest you merge.