Slider component

```
ui_component('slider', properties: { aspectRatio: '16x9' })
```

![slider](https://cloud.githubusercontent.com/assets/271645/3256527/a9b65af4-f219-11e3-9ae6-165ecd73dd85.gif)

- Responsive by default
- Lazy loading an option
- More opinionated than before
- Deprecates media gallery slider, hard

https://github.com/lonelyplanet/waldorf/pull/1279