Re: comments in #844 : "KeyValueStore is an XBlock interface, and is deliberately smaller than the dictionary interface"

I'm not necessarily suggesting that all the dict methods be implemented, but there are some things I don't understand -- why not an optional default for `get`? Why not having the superclass have a default `__contains__`? And I see that `items` might be an issue in some subclasses, but it's also probably a good decision to make early, since the first time anyone wants to use it, it's going to involve annoying code changes.