Ora2 aggregated data download button and management command

This is a pull request for an Ora2 aggregated data download button.  It is located in the Data Download section of the Instructor tab in edX.  It runs a raw SQL query located in instructor.utils (formerly located here https://gist.github.com/wedaly/f17ba1d2fffae65545c8) against the database.  This request also includes an associated management command, which performs the same query against the database, and can save the output file to an optionally specified directory or print it to stdout.

It would be nice if you guys could take a look at it and make suggestions before I attempt the upstream PR.  @jrbl @stvstnfrd @jbau @kluo 
can you mimic the logic in https://github.com/Stanford-Online/edx-platform/blob/master/common/djangoapps/util/query.py (or better yet, add to) to have the connection first try `db.connections['read_replica']`, and if that doesn't work, fall back to `db.connection`?

So what I'm picturing is that you add code to `query.py` that's something like `get_read_replica_cursor()` that encapsulates all that logic.