Renamed Mixins to Acts to resemble the change from include ActiveRecord::Mixins::List to acts_as_list and renamed @@default_error_messagess to @@default_error_messages

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@190 5ecf4fe2-1ee6-0310-87b1-e25e094e27de