Add ORM overrides to implement "track_changed_through" for a has_many through relationship.
Update the Item/Tag relationships for K3.
- add has_many through relationship to Tag and Item models.
- revise Model_Tag and Tag to use relationship.
- revise Tag model to use "track_changed_through" functionality.
- revise add() and remove() syntax.
- clean up when "item_related_update" is called.
Merge pull request #295 from shadlaws/orm_tag_item

ORM, Tag, Item