mostly remove the svg onClick hack

- chrome no longer throws a security warning when trying to read pixel
  data on a canvas after drawing *any* svg; only svgs with a <foreignObject>
  tag in them
- Standard Safari is still an old enough WebKit that it has this problem,
  but WebKit-nightly (SVN r173347) does not.
- instead of the blanket check, just try it and catch errors
Merge pull request #83 from chad3814/master

mostly remove the svg onClick hack