Resizing now done by ffmpeg not imagemagick; slightly worse quality but much faster
Optimized for performance, size, quality of input demo.mov.

* default quantize fuzz to 2%, expose as option
* default framerate 5 instead of 10
* add contrast increase, can suppress as option
* width/height scaling done by ffmpeg, helps speed and memory
* using imagemagick's "-layers OptimizeTransparency"
* trigger GC manually; see http://stackoverflow.com/q/958681/9621