Essentials of the fitting process for "accuracy model 3.0" using random features

Summary:
Code to generate the models used by webapp:exercises/knowledge_*. For an
overview of how the pieces here fit together, start with the doctring in
accuracy_model_featureset.py.

Test Plan: I ran the steps listed in accuracy_model_featureset.py.

Reviewers: mattfaus, robert

Reviewed By: mattfaus

CC: eliana

Differential Revision: http://phabricator.khanacademy.org/D2366