Upgrade to React 0.4, fully compliant with specs

This update puts the React Todo out of the labs since it's 100% compliant with the specifications. The transformed version (`jsx --extension jsx ./ ./`) also passes jshint. Every code style is respected also.

(For those who are interested, here's a list of all the changes:)

- Updated README
- Compliant with coding style.
- Removed app.css. No need anymore since we freed id.
- Render info footer separately (specs).
- Tweak footer to make it pass jshint when compiled:
  - {''} after 'left'.
  - {''} around `clear completed`.
  - Breaking style for footer clearButton, `>` on the same line.
- Rename cx and put it in Utils.
- Routing with director.
- Change some keyUp to keyDown.
- Strip form tags.
- Trim after editing item.
- Manually autofocus (attribute not supported in ie9).
- Input focus now triggers as a callback (because of batch rendering).
- Controlled input.
- Gutter at 80.
- Class `completed` trim whitespace.
- Remove bind.
- Jshinted.