WIP: Classy

* Add classy AI algorithm
* Improve the AI benchmark script
* Provide a Django cache to AI algorithms and use it to cache deserialized classifier data.
* Add AI grading requirements, including EASE
* Use wheels to install AI grading requirements
* Silence warnings

Most of the changes in this PR are for running tests that rely on scikit/nltk/scipy/numpy in Travis within a reasonable amount of time.

I'm relying on a newer version of scikit learn than EASE does, both because it's cleaner (`FeatureUnion` was added in version 0.13) and slightly faster.

Still to do:
* Many more test cases for Classy.
* Additional validation rules for when training examples don't contain at least one positive/negative example.
* Validate Classy using additional test essays.

@stephensanchez If you have time, feel free to start reviewing.