Rails 4

Hey everyone! Active Admin should now be ready for use with Rails 4. Previously you had to specify multiple gems to use the latest code on their repo, but now Active Admin is the only one that doesn't have an official release. For now, this is all you need to specify:

```ruby
gem 'activeadmin', github: 'gregbell/active_admin', branch: 'rails4'
```

Note that Rails 4 introduces a new API for [mass-assignment protection](https://github.com/rails/strong_parameters). You should use this pattern for every model:
```ruby
ActiveAdmin.register AdminUser do
  controller do
    def permitted_params
      params.permit admin_user: [:email, :password, :password_confirmation]
    end
  end
end
```

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: