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.
Currently, you have assets stored in a list. It seems like storing them in a dictionary by filename might make this function easier to work with. Is there a reason you didn't do it that way?

Alternately, it looks like this mongodb operator  http://docs.mongodb.org/manual/reference/operator/update/positional/ might make it possible to do the update in the database (although I'm not sure whether it can handle an insert).