Move CountingProbDist to learning.py and use it in NaiveBayesLearner. Make NaiveBayesLearner use target-value frequencies too.

Don't compare strings with 'is not'. (Broke things in pypy.) Also show Struct members in sorted order so doctests work in pypy.

Replaced Learner with a functional interface. This is a pretty big change you might especially want to review.

Fixed DataSet doc comments. Added rep-invariant check. DecisionTreeLearner now breaks ties on leaves randomly. NaiveBayesLearner now collects stats only on input fields. Minor cleanup.

Lined up DataSet params with the documented fields. Documented extra param. Fixed .sanitize() to replace with None (the old code no longer did quite what was documented).