copy r13239 awesome_nested_set modification to Issue model

git-svn-id: http://svn.redmine.org/redmine@13290 e93f8b46-1217-0410-a6f0-8f06a7374b81
revert r13239 awesome_nested_set modification

git-svn-id: http://svn.redmine.org/redmine@13291 e93f8b46-1217-0410-a6f0-8f06a7374b81
Gemfile: use awesome_nested_set gem

git-svn-id: http://svn.redmine.org/redmine@13292 e93f8b46-1217-0410-a6f0-8f06a7374b81