Show environment indicator

* Upgrade to latest version of admin gem
* 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.
* This requires some initialisers to be set at deploy time (see Publisher example)
* Highlight environment using a favicon, a coloured navigation bar and a coloured label
* Create a custom [S] support favicon

![screen shot 2014-06-05 at 17 17 12](https://cloud.githubusercontent.com/assets/319055/3190316/030377a6-eccd-11e3-8c48-ec326a3935c4.png)