Issue #38: Embed an editor for trying things on sweetjs.org

Hello,

I've integrated CodeMirror (http://codemirror.net/) in index.html, so users can try to macro expand their own snippets.

You can see it at work here: http://voila.github.com/sweet.js/

I've added expander.js, parser.js and sweet.js to browser/scripts, in order to get it to work. Hope that's all right.