Add a `picture` component

I'm opening this PR early to try get some discussion happening around the implementation I've gone for.

![picture-component](https://cloud.githubusercontent.com/assets/708296/3890508/75a031c8-221f-11e4-8da1-dc53dfc7b510.png)

Having said that, do you think it's enough to link to this? http://scottjehl.github.io/picturefill/#ie9