Codemirror

This is a first step to make examples interactive.

- Should we get codemirror stuff from CDN? http://cdnjs.com/libraries/codemirror/
- It would be nice to "rerun" examples, I'm thinking how to do it. (One idea: Always copy the example related DOM part, if someone messes with global state,, than it wouldn't work, but why anyone would do that). Any other ideas?