Refactor media gallery

![media-gallery](https://cloud.githubusercontent.com/assets/271645/3312977/fd76fec0-f6db-11e3-8ffc-84a847de7fe2.gif)

- Moves media gallery Sass into Waldorf, yay!
- POI list JS component scrapes data from markup rather than a single JSON encoded attribute
- Adds reset/re-centre methods

__This is a breaking change, apps which accidentally include the `_media-gallery.sass` partial will explode__
Maybe rename it too if you can make it clearer. They're all global.