check in the MIRT EM-based training

Summary:
The main goal here is to get the MIRT traning into the repo.  We have had at least two
versions.  For right now, I am only checking in the EM version.  This goes along with
Kiln review https://khanacademy.kilnhg.com/Review/K113501 .  The traing here must
now import the website repo to use the shared code in mirt_util.py

On a lesser note, I also stopped outputting defaultdicts to file, which was the
cause of much headaches.

Test Plan:
I have run the training, and the used the interactive simulation mode
of mirt_engine.py (in the website repo) to test.

Reviewers: jascha, james

Reviewed By: jascha

CC: chris

Differential Revision: http://phabricator.khanacademy.org/D2086