Asynchronous download button for ORA2 data

This pull request implements a management command and instructor dashboard button which gather all open assessment response data for a given course in a user friendly format and output it as a .csv.  It does so by submitting a task to the celery queue and waiting for it to upload the result to S3 in an asynchronous fashion.  Data collection is performed via a templatized, raw SQL query (admittedly a bit unorthodox), the original version of which may be found <a href="https://gist.github.com/wedaly/f17ba1d2fffae65545c8">here</a>.  Also included are unit tests for the functions added in various locations.  Feedback is welcome and encouraged @jbau @sarina as well as tagging more people who should have a look.  Thanks!
Why are you writing SQL directly, rather than going through the Django ORM? It's generally considered good practice to use the ORM where possible, especially since there are people in the community who want to port Open edX to run on a different RDBMS.