Ensure consistency in directional meaning of MIRT ability estimates-- higher should always be better.  Currently only works for single dimensional models.

Test Plan:
I ran mirt_train_EM.py over prod_responses in /ebs/kadata/accmodel/mirt on the analytics machine.
Visually inspected the .csv output for later epochs, and also ran the interactive simulator in mirt_engine.py.
Verified that correct answers were giving me higher ability estimates.
I also repeated all the above tests with temporarily inverted logic in this changeset, and everything worked
the same except the directional meaning.

Reviewers: eliana

CC: chris, jascha

Differential Revision: http://phabricator.khanacademy.org/D2183