Add support for global <style>, as in Illustrator's SVG output (#12)

[edemaine]

This is actually pretty convenient using getComputedStyle, or jQuery's interface .css(). We just need to add the SVG to the document's DOM briefly (but remove it before the browser has a chance to render). I still kept the old lookup methods as fallbacks, in particular in case we're running outside of a browser in the future.

I also fixed the handling of simultaneous opacity and stroke-opacity. According to some experiments in Chrome and some random webpages, these get multiplied together if both occur.

[edemaine]

Reading over "File Import Tips" for the nth time, I realize there was another way to use Illustrator -- I didn't realize I could tweak how styles were set in saved SVG files (quite handy!). But it's no longer needed for Origami Simulator (according to some limited testing), so I wonder whether we should delete that bullet and the two associated images?