Minor Changes in ReadMe

Fix import error in utils.py and compile error in mdp.py

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

 * Use weighted_sampler() for the sampling.
 * By not using a defaultdict, keep n_obs always up to date.
 * Don't inherit from ProbDist, at least for now, since we don't
   use anything from the superclass, and if we did it would break
   (e.g. __setitem__).
 * Remove __len__ since nobody uses it.
 * Use heapq.nlargest.

Also separate random doctests from others, and tweak docs.

Remove astar row from compare_graph_searchers table because it's sensitive to trivial changes. Needs real tests instead.

 * graph_search missed deduping nodes in the frontier
 * astar_search didn't use 3rd-edition pseudocode
 * InstrumentedProblem didn't forward the path_cost method,
   so all the doctest results on instrumented cost-sensitive
   problems were wrong

Also filled in breadth_first_search and per-node memoization 
of h (to fulfill the doc comment saying that was done).

There's a remaining problem that sometimes trivial refactorings
change the amount of search that is done, slightly, which should
not happen since the code is supposed to be deterministic.