Update to native <picture> syntax (Picturefill 2.0)

Tested against Picturefill 2.0.0-beta and 2.1.0-beta.

I've messed with on my own site and using the code in `examples` but I'd love for someone else to give this a go to see if I missed a feature. It works in Firefox, Safari, Chrome, and Chrome Canary (which uses native `picture` rather than the polyfill).

A few future updates this doesn't include:
* Include the IE9 `<video>` hack, as show in http://scottjehl.github.io/picturefill
* Provide a `srcset/sizes` option in addition to `<picture>`.