Add mgmt cmd to generate anonymized ID mapping

So that instructors have easy access to the mapping from anonymized
IDs (a simple MD5 hash of the user ID that's used when integrating
with third-party services like Qualtrics) to user IDs, we have a
simple Django management command to generate a CSV mapping. To run,
use the following:

rake django-admin[anonymized_id_mapping,<system>,<env>,<course_id>]

And, of course, substitute the appropriate values for <system>, <env>,
and <course_id>. (E.g., lms, dev, and MITx/6.002x/Circuits)
Anonymized ID mapping fixups

Fixups to Nate's anonymized id mapper, repairing pep8 and pylint errors,
and rebasing on recent master.
Merge pull request #663 from edx/nate/anonymized_id_mapping

Add mgmt cmd to generate anonymized ID mapping