import awesome_nested_set 2.1.6

https://github.com/collectiveidea/awesome_nested_set/commit/64cc8bc8cf93e8859a21bcf25c36675080e42ce6

git-svn-id: http://svn.redmine.org/redmine/trunk@12687 e93f8b46-1217-0410-a6f0-8f06a7374b81
add lib/plugins/awesome_nested_set/init.rb

git-svn-id: http://svn.redmine.org/redmine/trunk@12688 e93f8b46-1217-0410-a6f0-8f06a7374b81