Makefile: Add an image-conversion makefile

Makefile: Add an image-conversion makefile

Not all images will be SVGs.  This Makefile handles converting other
formats to consistently sized PNGs, so you don't have to do the
conversion in your browser during the presentation itself.  The
Makefile is adapted from the one I developed for my thesis defense
[1], as of 3fdff3b (index.html.itex: List new software in conclusions,
2013-05-28), and is licensed under the same MIT license Hakim uses for
Reveal.js.

[1]: http://git.tremily.us/?p=reveal.js.git;a=blob;f=Makefile;hb=refs/heads/thesis