add Page#namespace_name

Page#namespace_name returns the name of the page namespace. This is usefull for cancan abilities for namespaced pages.
```
ActiveAdmin.register_page "Dashboard", namespace: "foo"

class Ability
  include CanCan::Ability

  def initialize(user)
    can :read, ActiveAdmin::Page, :name => "Dashboard", namespace_name: "foo"
  end
end
```

This closes #2854
change authorization docu for namespaced pages
change docs hash style to 1.9 syntax