Possibility of typing string values more tightly

[daboross]

Right now, methods like owner_name and id return strings. We could be strictly more accurate and return wrappers around String, like Username and ObjectId.

My main reasoning not to do this is that it'll all be serialized and deserialized in between ticks anyways, but that's not a very strong argument.

[ASalvail]

Partially addressed by #229