Improve Delete Exhibit callout and button styling

Fixes #590 

![administration_-_settings___default_exhibit_-_blacklight](https://f.cloud.github.com/assets/101482/2507275/86ac0d6a-b3b7-11e3-96ca-7e72b43bbbad.png)

And on delete button hover/focus:

![administration_-_settings___default_exhibit_-_blacklight 2](https://f.cloud.github.com/assets/101482/2507279/a1c16488-b3b7-11e3-86d0-443c9a0f1f85.png)