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 [T] transition favicon

![screen shot 2014-06-05 at 12 34 02](https://cloud.githubusercontent.com/assets/319055/3186626/590d036a-eca5-11e3-846c-5b41db7ac4b3.png)