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)

A helper to create this slug could be quite nice. Would eventually fit with this card though: https://trello.com/c/NvEaotQ8/23-images-move-resrc-it-rubberband-classes-into-rizzo. No rush on it