Refactored query for speed.

Now queries first for a list of distinct bus IDs, then loops over
IDs find most recent timestamp for each. Need to handle the
"list of distinct IDs" query separately -- ideally in separate
MongoDB collection.
Added file explaining usage.
Merge branch 'master' of https://github.com/abeboparebop/mongodb-to-onebusaway-gtfs-realtime-feed