Add Media component with tests.

Add the classic [Media Object](http://www.stubbornella.org/content/2010/06/25/the-media-object-saves-hundreds-of-lines-of-code/), implemented using Flexbox.

Status: **Opened for visibility**
Reviewers: @jeffkamo @kpeatt @scalvert 

## Changes
- Add Media component
- Add tests

## Notes
- This could be built using the Arrange component. However, since it’s so common, so useful and so well-defined, a stand-alone, separately-tested implementation is valuable.