add a utiility for dealing with MIRT models that can convert from npz to JSON and also upload to GAE through the API

Summary: This is the script that can be used to create and upload a MIRT model and create a new AssessmentParams entity

Test Plan: after generating the access token, i tested locally, and also used to upload to production.

Reviewers: chris

Reviewed By: chris

CC: jascha

Differential Revision: http://phabricator.khanacademy.org/D2037