Added Picturefill

1. The original file is here: https://github.com/scottjehl/picturefill/tree/master/dist

2. I failed to do the `npm test` thing. Not sure how to get it to work. Here's a screenshot:
![capture](https://cloud.githubusercontent.com/assets/716405/3067135/4a8f6114-e27f-11e3-9608-9090d1d57c9a.PNG)