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 [TA] favicon

![screen shot 2014-06-05 at 17 55 07](https://cloud.githubusercontent.com/assets/319055/3190730/2e5768f4-ecd2-11e3-8cc6-47cbb8e054ae.png)

If we’re adding this gem to apps that we’re going to put live, would you be happy bumping it to `1.0.0`? Is it stable enough that we’re happy supporting it?