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 [M] Maslow favicon

![screen shot 2014-06-05 at 13 07 31](https://cloud.githubusercontent.com/assets/319055/3186903/2fc10f42-ecaa-11e3-8ac7-1df7d646db7f.png)