UX review

Pull request for issue #93 

Changes:

* Use 100% width
* Opens script editor by default, and places it beside the readme
* Makes readme editable

![explorer](https://f.cloud.github.com/assets/8142/401689/7b09c2e6-a8e5-11e2-946e-725f894a7365.png)

I don't think this work is finished, but it's better than what we have now, so I think we should merge early and iterate.

Things I'd still like to do:

- [ ] Try moving readme to full-height column on the left
- [ ] Add jsfiddle-style panel resizing
- [ ] Add the ability to insert/delete views