Remove count_if, find_if, every, some, Struct; replace them with modern
Python equivalents.

- Fix relative imports in base of is a package or not
- Fix not working notebooks
- Add notebooks to packages sources

Used tool 2to3 to finish the porting to Python 3 plus some fixes by
hand.

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.