Auto compile

Hi, 

really enjoy experimenting with Elm, and I like your learn-by-example / no-install approach (although installing Elm locally was straight forward as well).

While working with the online examples and trying to tweak them I found myself going through the documentation several times to try and find what I was looking for. So I want to see if some of CodeMirrors features can be used to make it even easier to experiment with and learn Elm.

This is a small step which includes two features:
* keyboard shortcut to compile: use Ctrl+Enter in the editor to update the output pane
* auto compile: automatically update the output frame when user pauses typing. This is off by default.

The pull request also updates CodeMirror to version 3.0, as I think it is best to work from the latest release when venturing to other things, like inline documentation (which I wanted to take a stab at).