Add a script to train MIRT models for assessments

Summary:
A simple automation of the instructions Jace sent me via email for
building/training MIRT models.

This makes mirt_npz_to_json.py scriptable by having the interaction take
place on stderr instead of stdout, otherwise it mixes with the script's
output.

Some comments moved to command line flag descriptions in
mirt_train_EM.py. Not needed by this implementation, but useful for me
in figuring out what mirt_train_EM.py does.

Test Plan:
Ran dt=2013-03-11 ./train_mirt_models.sh and verified that reasonable
looking data was obtained as a JSON model.

Reviewers: jace

Reviewed By: jace

Differential Revision: http://phabricator.khanacademy.org/D2149