Fallback to file-based cache for classifier data
Add info-level logging for cache hits/misses
Merge pull request #456 from edx/will/ai-cache-classifiers-to-disk

AI: Celery workers lose their cache after an exception causing many hits to S3
Changed NLTK requirements from 2.0.3 to 2.0.4
Update wheel archive for NLTK
Merge pull request #461 from edx/grady/NLTK-requirement-change

Changed NLTK requirements from 2.0.3 to 2.0.4
Allowing file based submissions for ORA2