Put tutorial code up-to-date

- Use React 0.10
- Reach line-by-line parity with tutorial code.
- Use showdown CDN like in the tutorial.
- New jQuery version to supress the weird "JSON syntax error" from 1.10.
- Fix a rare race condition where the optimistically updated comments is set after the ajax request succeeds.
- Lock express version (new one drops Connect, so the `node server.js` command fails.