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 
So what happens when ```data_list``` is the empty list?

This returns ```None```, and then we call ```writer.writerow(None)```

does that behave the way we expect? Or does it cast None to a string and write out the literal value ```"None"```?

What happens when ```rows``` contains a mix of valid rows and empty rows?