Rails 4

Hey everyone! Active Admin should now be ready for use with Rails 4. On top of the regular steps to migrate from Rails 3 to 4, you need to specify these gems:

```ruby
gem 'activeadmin',         github: 'gregbell/active_admin', branch: 'rails4'
gem 'ransack',             github: 'ernie/ransack',         branch: 'rails-4'
gem 'inherited_resources', github: 'josevalim/inherited_resources'
gem 'formtastic',          github: 'justinfrench/formtastic'
```

If you have any troubles, please post here if it's something small, or otherwise open a new Issue.

Thanks to everyone who helped identify issues in #1963, and especially thank you to everyone who submitted code :heart: