Simple HTML to JSX converter

Simple HTML to JSX converter, built during Hackathon 40 at Facebook. See /react/html-jsx.html. Not directly linked from the site yet as there may still be some minor issues with it.

I'll eventually split `html-jsx-lib.js` into another project (react-tools?) as it could be used elsewhere. I think it'd work with Node.js if a module like jsdom was used to implement the DOM functionality.

*This was originally sent in #667, but this time it's been branched off master instead of 0.5-stable.*