create user_tags model and taggings join table
associate user_tags with stories and playlists
add To Do item to user_tag model
add uniq to tags method to avoid repeated tags
change scope of uniqueness constraint on taggings so that a story and a playlist with the same ID could have the same tag
move the uniq call in story.tags to a more logical place
Merge pull request #52 from PRX/user_tags

User tags