Reworked tagging support

git-svn-id: https://plugins.svn.wordpress.org/ghost/trunk@795062 b8457f37-d9ea-0310-8a92-e5e31aec5664
fixed duplicate tag/category slug issue

git-svn-id: https://plugins.svn.wordpress.org/ghost/trunk@795086 b8457f37-d9ea-0310-8a92-e5e31aec5664