Add ORM::delete() override to "delete_through" of a has_many relationship.
Update Group and User definitions, models, etc.
- implement K3's has_many through relationship to work with groups_users
- add groups() and users() to the interface defs (the code base effectively
requires them already)
- revise add() and remove() syntax for K3
- remove user_cache and group_cache (ORM and Database cover this for us)
- revise delete() based on the new ORM::delete() override
- revise some instances of save/update/create syntax
- make Identity use groups() as opposed to the model relationship directly
(compatible with interface requirement and agnostic to DB configuration)
Merge pull request #293 from shadlaws/orm_user_group

ORM, User, and  Group