Fixed pruning for first max node in aphabeta_search

Replaced Calls to if_ with Pythonic Syntax

Fixes Issue #29.

These functions, which I had to implement in Python 2.2, are now
available in standard Python.

modifications in README.md on format of tables

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