In the precondition positive list, I am not sure whether b!=x is the best way to represent the condition. IsNot(b, x) can be used instead.

* Minor docstring changes

* Added Spare Tire Problem

* Fixed a bug in substitute method of class Action

* Fixed minor typo in comment

* Added Action class for PDDL

* Added tests for Action class

* Changed doc string

* Added PDLL class

* Added Air-Cargo-problem

* Tested Air Cargo Problem

* Made Log Usage Consistent in text.py

* Added Explicit Imports from utils

Used tool 2to3 to finish the porting to Python 3 plus some fixes by
hand.