BUMPER PAK!

Today's changes:
1. For now, use a dummy cache for testing.  We will want to be able to test cache behaviour at some point.
2. Use sets instead of lists for multiple metadata.
3. Allow query date to be None; this signifies a query for the current state of the metadata and allows this
   to be cached properly.
4. Some tidying up here and there.
5. Return only the metadata values from the database, instead of taking the whole model and promptly throwing
   it away.