Generate a gallery of animation comparions

We show the SVG animation on the left and the equivalent animation using
Web Animations on the right.

At the bottom of the page, we show the source code for each animation.

The gallery generated by this change can be visited at
http://svg-animation-gallery.appspot.com/static/generate-gallery/svg-animation/examples/index.html
I'd call this file generate_index.py.