Add Ratio component.

Add component for easily constraining fluid elements to a particular aspect ratio. Most useful for things [like video](http://alistapart.com/article/creating-intrinsic-ratios-for-video/), the pattern can be useful in other situations as well.

Status: **Opened for visibility**
Reviewers: @kpeatt @jeffkamo @scalvert

## Changes
- Add ratio component.

## Notes
- Needs tests.