Menu mapper: add support for :before, :after and :last options to #push method...

Menu mapper: add support for :before, :after and :last options to #push method and add #delete method.

git-svn-id: http://redmine.rubyforge.org/svn/trunk@1660 e93f8b46-1217-0410-a6f0-8f06a7374b81