Added parent-child relationship to pages to visualize a breadcrump and subpages.

vendor/plugins/acts_as_tree/README

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+acts_as_tree
+============
+
+Specify this +acts_as+ extension if you want to model a tree structure by providing a parent association and a children
+association. This requires that you have a foreign key column, which by default is called +parent_id+.
+
+  class Category < ActiveRecord::Base
+    acts_as_tree :order => "name"
+  end
+
+  Example:
+  root
+   \_ child1
+        \_ subchild1
+        \_ subchild2
+
+  root      = Category.create("name" => "root")
+  child1    = root.children.create("name" => "child1")
+  subchild1 = child1.children.create("name" => "subchild1")
+
+  root.parent   # => nil
+  child1.parent # => root
+  root.children # => [child1]
+  root.children.first.children.first # => subchild1
+
+Copyright (c) 2007 David Heinemeier Hansson, released under the MIT license