Ihoover/automated content testing

Framework for course authors to create tests for the problems they author.  This PR has just the framework laid, and the purpose is to get this code reviewed before moving on. All urls associated are disabled by the feature flag `MITX_FEATURES['content_testing']`.

To use this code, first go to `cms/envs/common.py` and modify line 64 to read ``'CONTENT_TESTING': True,``.  This enables the url paths `/test_problem/...`.  To use the code, run `rake cms` and navigate to `/test_problem/?location=<location>`, where `<location>` is the location of a problem you have the authorization to edit.
@jzoldak @wedaly