add gifsicle support

[twobutts]

gifsicle is much better at gif manipulation than imagemagick. our current gif thumbnail generation is actually regressive.

http://www.lcdf.org/gifsicle/

gifsicle --unoptimize --resize-fit=300x300 --color-method=median-cut --colors=64 --optimize=3 < in.gif > out.gif