MongoLike groupby ignore sort, skip, limit kwargs; confusing properties behavior

[rkingsbury]

Thanks to clever synatx highlighting in my code editor, I noticed that the kwargs sort, skip, and limit that are shown for MongoStore.groupby and MemoryStore.groupby appear to be non-functional.

These kwargs should be made functional (preferably, if it makes sense) or removed.

Related - it is not clear to me why the groupby methods for MongoStore and MemoryStore need to be different, since MemoryStore is designed to emulate MongoStore in every respect. If MemoryStore could inherit MongoStore's groupby that would be more robust (and prevent subtle bugs like the one described in #621 ).

Related #2 - MemoryStore.groupby does not return any data unless you explicitly pass fields to properties. This may affect other MongoLike stores, too but I'm not sure. See discussion here.

I may investigate this at a later time but would welcome comments from better-informed developers!