Added multi-button-group and moved button-group widgets to use `content`

Summary:
MultiButtonGroup allows you to select multiple buttons in your button group.
I've added an example to the documentation that also demonstrates how the button
content field can be arbitrary JSX, not just text. Because MultiButtonGroup and ButtonGroup
use common styles, I also added a styles.js file that can be used to add and require common
styles. I think this choice will be subject to some change, but it's a starting point.

Test Plan: Tested extensively in Perseus for various use cases.

Reviewers: joel

Reviewed By: joel

Differential Revision: http://phabricator.khanacademy.org/D10987