Add best practices for contributions

TODO (completed): Move >>> doctests from utils.py to utils_test.py

removed redundant function definition for standard dev

modified print statements in agents.py to make it work in Py3, changed utils_test.py with regard to new utils.py

Fix for #61

Update utils.py

Created grid.py from utils.py for separated 2D Grid environment.

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.