OpCompressedCache: fixed comment.
Added scikitClassifiers.py, which contains classifier operators with swappable implementation.
Defined classifier interface and replaced old RF classifierOperators with generic versions.
classifiers: Refactored again.  Distinction between 'classifier' abc and 'classifier factory' abc.
classifier operators: classifier factory is configured as a slot, not a constructor arg
classifiers: Added serialization
classifiers: Added parallel RF classifier (the original classifier implementation)
OpCompressedCache: fixed comment.
Merge branch 'scikit-classifier'