Anonymized ID mapping fixups

Fixups to Nate's anonymized id mapper, repairing pep8 and pylint errors,
and rebasing on recent master.

common/djangoapps/student/management/commands/anonymized_id_mapping.py

+# -*- coding: utf8 -*-
+"""Dump username,unique_id_for_user pairs as CSV.
+
+Give instructors easy access to the mapping from anonymized IDs to user IDs
+with a simple Django management command to generate a CSV mapping. To run, use
+the following:
+
+rake django-admin[anonymized_id_mapping,x,y,z]
+
+[Naturally, substitute the appropriate values for x, y, and z. (I.e.,
+ lms, dev, and MITx/6.002x/Circuits)]"""
+
 import csv
-import sys
 
 from django.contrib.auth.models import User
 from django.core.management.base import BaseCommand, CommandError
 
 from student.models import unique_id_for_user
 
+
 class Command(BaseCommand):
+    """Add our handler to the space where django-admin looks up commands."""
+
     # It appears that with the way Rake invokes these commands, we can't
     # have more than one arg passed through...annoying.
     args = ("course_id", )
 
-    help = """
-    Exports a CSV document mapping from a username to the anonymized,
-    unique user ID for every user in the specified course.
+    help = """Export a CSV mapping usernames to anonymized ids
+
+    Exports a CSV document mapping each username in the specified course to
+    the anonymized, unique user ID.
     """
 
     def handle(self, *args, **options):