Use SVG defaults for stroke-width, fill and stroke attributes

First of all, thank you for this great piece of code!

I had a small issue with the default value of stroke-width set to 0. According to http://www.w3.org/TR/SVG/painting.html it should default to 1, but was 0 because of some old bug in Chrome (at least that what I could understand from git log). With the default setting all tests pass and visuals match too, at least in FF and Chromium. Alternatively, I'd consider removing these default altogether, but maybe that's a good idea to force them in all browsers.