Dropdown actions for index

When adding many custom actions using index as table you get a large column for actions.  This adds a 'dropdown' option to actions method in order to display the actions in a dropdown menu

Some Examples:

To list default actions and customs actions:
```ruby
index do
  selectable_column
  column :title
  actions dropdown: true do |post|
    item "Preview", admin_preview_post_path(post)
  end
end
```

To list custom actions without default_actions:

```ruby
index do
  selectable_column
  column :title
  actions dropdown: true, default: false do |post|
    item "Preview", admin_preview_post_path(post)
  end
end
```

Preview:

![screen shot 2013-12-06 at 10 51 00 am](https://f.cloud.github.com/assets/232293/1692526/933f76be-5e7d-11e3-9a31-b463f8d1ef23.png)