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.
1) You can leave out the numbers inside the braces (they're implict, unless you're repeating them)
2) Usually I'd format this by leaving `.format` on this line, and then having all of the parameters inside the parens w/o  the trailing `\`