Add production SVGs, which address issues #6 and #37

These SVGs have been further optimized for size (whitespace removed), and
removed any non-rendering paths.

The svg/design subdirectories continue to include a non-rendering path outlining
the border of the icon, which is useful for designers when dropping these icons
into tools like Adobe Illustrator or Sketch
Update index for SVG move
Change our license from CC-BY-SA, to CC-BY

We originally licensed these icons under CC-BY-SA because we wanted to make them
available to you to incorporate into your apps, but we didn't want someone to be
able to package them up and resell them as an icon set.  We heard your concerns
about the scope of the share alike obligation.  Our primary goal here is to make
it easy for you to use the icons, so we've changed the license to CC-BY.
Regenerate spriets and fix bug making it hard to use multiple CSS/PNG sprites
together.