Phase 1 of adding asset metadata saving to old Mongo

@dmitchell @cpennington Here's the first phase of my work. Look at the tests to see what is implemented and what isn't yet.
I know this is how it's done in the ContentStore code, but I think we'd be better served in the long run to use a class for this, either defined explicitly, or by just using `namedtuple` (https://docs.python.org/2/library/collections.html#collections.namedtuple)