Add Makefile & update PNGs

The Makefile uses optipng to avoid gratuitously large PNGs.  This might work better without commit 1f438784ce (which added white backgrounds); at least, optipng had a lot more success in converting images to 8-bit paletted form before I re-generated the PNGs using rsvg-convert (i.e. regenerating increased the number of colors in the images).  `logo-mime.png` does so badly that it actually gets *bigger*.

Unfortunately, rsvg does not appear to grok `logo-xhr` too well; one of the strokes has gone quite wild, so you should not pull this yet unless you know how to deal with that.  (Perhaps it works in Inkscape, or it is easy to work around the problem in rsvg, or ...)

[Just sending this now so I don't have to worry about my browser eating it.]