Implement Live Preview in Shortcake UI

This branch adds a live preview to the Shortcake UI, per #45. The preview is currently un-styled, appearing at the top of the UI (under the 'Edit' title).

![screen shot 2014-11-24 at 7 19 04 am](https://cloud.githubusercontent.com/assets/1703671/5167077/4f1dd8fc-73aa-11e4-8de8-e6b8658b8254.png)

This implementation is not yet tested against more advanced shortcodes or Shortcake inputs other than &lt;input> and &lt;textarea>. I'm opening this pull request primarily to facilitate review of current progress and discussion of next steps.

Note that this feature is impacted by the missing attributes referenced in #60.
> Should we namespace this?

Yes. Because the CSS is not only loaded in the preview &lt;iframe> (sandbox). It is also loaded in the editor where it could potentially conflict with other styles.