More useful `actions` method for `index_as_table`

Now you can pass any options which ```column``` supports.
For example:
```ruby
ActiveAdmin.register Post do
  index do
    actions I18n.t('active_admin.edit'), :defaults => false, :class => "col-edit" do |post|
      link_to(I18n.t('active_admin.delete'), admin_post_path(post), :method => :delete, :data => {:confirm => I18n.t('active_admin.delete_confirmation')}, :class => "member_link delete_link")
    end
  end
end

```