Prevent double-initialization of TagManager

If you call $().tagManager() more than once on the same object, it will double-initialize and cause strange behavior like duplicated tags, deleting multiple tags on each backspace, etc. I've added a feature that prevents this, but still allows you to run the pop/push commands as normal.

In some apps where Javascript is used heavily, it may be hard to avoid double-calling tagManager(), and moreover, this pattern is used by many of the best jQuery plugins. See: http://stackoverflow.com/questions/11066214/stop-jquery-plugin-applying-twice-to-same-element