Distinguish between Publisher environments

* Distinguish between Preview and Production so that users don't accidentally make changes to Production content whilst under the assumption they're working on Preview.
* Highlight environment using a favicon, a coloured navigation bar and a coloured label
* Determine environment using `Plek.current.website_root`
* User "preview" and "production" terminology for now, "live" and "test" may be better suited

![screen shot 2014-06-02 at 13 03 05](https://cloud.githubusercontent.com/assets/319055/3146833/e81f6caa-ea4d-11e3-8a90-e2ad171b858a.png)

![screen shot 2014-06-02 at 13 03 32](https://cloud.githubusercontent.com/assets/319055/3146836/f5eaa246-ea4d-11e3-8f89-0c9aa9ebb77c.png)

![screen shot 2014-06-02 at 13 06 47](https://cloud.githubusercontent.com/assets/319055/3146867/73098d3c-ea4e-11e3-8ad8-bcdf2f2c8ef9.png)