UI: make editor height responsive to DevTools resize, put button above editor, remove default body padding, fixes #9

See #9 

This PR includes:

- CodeMirror editor height is now responsive to DevTools resize (`.CodeMirror { height: auto; }` and removed `setSize` call).
- "Run it" button stays above the editor (`z-index`). Changed `absolute` to `fixed` as what you really want is `fixed`, and `absolute` was relying on buggy behavior.
- Removed unnecessary body margin -- the scrollbar looks better attached to the outer border, also provides a bit more of room to the editor.

End result:

![](http://i.imgur.com/AWVucoR.png)