Default sort order should use primary_key (not id)

[gregbell]

Currently, if you customize the primary key in an Active Record object, the sorting will raise an exception.

Instead of using a global #default_sort_order, it should get one from the model

  # lib/active_admin/resource.rb
    def default_options
      {
        :namespace  => ActiveAdmin.application.default_namespace,
        :sort_order => ActiveAdmin.application.default_sort_order
      }
    end