Improve rendering of examples and composition of React components

This adds a tabbed interface to the various source code snippets that go into making up an example. Overall these visual changes make it easier to view an example's source and output as a whole unit.

This also adds `propTypes` to all React components which make them easier to use and debug. The `LocaleSelect` component now also can work using React's `valueLink` feature.

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![screenshot 2014-09-29 11 46 25](https://cloud.githubusercontent.com/assets/29096/4444538/af89242e-47f1-11e4-8f04-fe07b800583a.png)