Multi-Level Menu

It would be nice to have support for a mulit-level menu. In the following scenario, it would be nice to see "Test3" show up as a sub menu under "Test2" rather than create a new top level menu item.

``` ruby
ActiveAdmin.register Test1 do
end

ActiveAdmin.register Test2 do
  menu :parent => "Test1"
end

ActiveAdmin.register Test3 do
  menu :parent => "Test2"
end
```
