This is really the pull request from tmrts, but as a novice git user, I couldn't figure out how to do the merge after there were some other changes, so I did it manually. Sorry that you don't get official credit, tmrts, but thanks!

Pull request for bug #37.

Fix for bug no. #38.

Add .gitignore

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.