Converted Menus to a Tree structure to allow submenus.

* Bundle the rubytree gem
* Patched RubyTree's TreeNode to add some additional methods.
* Converted the menu rendering to walk the Tree of MenuItems to render
  each item
* Added a menu option for :parent_menu to make this menu a child of the parent
* Added a bunch of tests
* Made MenuItem a subclass of Tree::TreeNode in order to use it's methods
  directly
* Changed the exceptions in MenuItem#new to be ArgumentErrors instead of the
  generic RuntimeError

  #4250

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@3090 e93f8b46-1217-0410-a6f0-8f06a7374b81