I'm comfortable with this change because `item` is always a couchdbkit-wrapped object, so for the common things, e.g. user_id, `case.user_id` and `case['user_id']` are equivalent.

The motivation for the change is that in Tyler's case he's subclassing to add `@property`s for easy access. Old couchdbkit will let you access an `@property` with doc['my_at_property'], but that seems like sloppy behavior; jsonobject only lets you use bracket notation to get things that are actually in the underlying dict. (Also it is called `AttributeGetter`, so it seems appropriate to expect it to use `getattr`.)

I also just did a project-wide search for AttributeGetter and group_by, and it all seems fine.