Added a test for animated WebP images

This tests for animated WebP using the data URI from https://developers.google.com/speed/webp/faq.

Did some quick testing:
- Firefox 26 beta: returns false for support
- Chrome 31: returns false for support, does not render animated webp on this page: http://blog.chromium.org/2013/11/chrome-32-beta-animated-webp-images-and.html
- Chrome 32 beta: returns true for support, renders that animated webp