Implemented polyfill for new <picture> spec

Hi everyone!

I spent a bit of time writing a polyfill for the new <picture> proposal
written by Tab Atkins (http://picture.responsiveimages.org/). Feel free to
check it out here: https://github.com/jansepar/picturefill. I was surprised
to see one hadn't been created yet so I decided to give it a go.

Any feedback is much appreciated!

~Shawn Jansepar