add a utility script that can convert UserAssessment data straight from S3 downloads into a format useable by mirt_train_EM.py (and other training routines)

Summary: Chris, I'm just getting this checked in, as I prepare an email for you on how to train the MIRT models.

Test Plan: I followed the steps in the docstring, and verified the output looked sane.

Reviewers: chris

Reviewed By: chris

CC: eliana

Differential Revision: http://phabricator.khanacademy.org/D2140