import awesome_nested_set 2.1.5

https://github.com/collectiveidea/awesome_nested_set/commit/60fe4f69e4d6c3052bb45fb3e0a118ca201d9054

git-svn-id: http://svn.redmine.org/redmine/trunk@12677 e93f8b46-1217-0410-a6f0-8f06a7374b81
add lib/plugins/awesome_nested_set/init.rb

git-svn-id: http://svn.redmine.org/redmine/trunk@12678 e93f8b46-1217-0410-a6f0-8f06a7374b81
apply Redmine awesome_nested_set 2.1.5 modification

git-svn-id: http://svn.redmine.org/redmine/trunk@12679 e93f8b46-1217-0410-a6f0-8f06a7374b81