Use govuk_admin_template

- Add gem to gemfile
- Include stylesheet in base.css
- Rewrite application.html.erb to use template

Ticket: https://trello.com/c/iE62ZEmf/144-explore-admin-gem-for-specialist-publisher

Before: 
![screen shot 2014-06-16 at 16 52 55](https://cloud.githubusercontent.com/assets/68009/3289596/56ac3c7c-f56e-11e3-80ff-c01ef52c6241.png)

After:
![screen shot 2014-06-16 at 16 53 03](https://cloud.githubusercontent.com/assets/68009/3289597/5b390dce-f56e-11e3-96c2-be4d9b3cc1a8.png)