use “raise NotImplementedError” for abstract methods

This is from tmrts, pull request https://github.com/tmrts/aima-python/commit/4cc647ae3e1d4ab9a1fc4fc9be652f52b4e72851 , but I'm bad at git, so it was easier for me to merge by creating this file here.

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.