Added test for spare_tire problem of planning module.

* Implementing HITS algorithm

* Moving HITS work to nlp.py and test_nlp.py

* Minor docstring changes

* Added Spare Tire Problem

* Fixed a bug in substitute method of class Action

* Fixed minor typo in comment

The link to "Pseudocode algorithm (pdf)" was pointing to https://github.com/aimacode/pseudocode/blob/master/algorithms.pdf, which was causing the 404 error. The name of the file algorithms.pdf was changed to aima3e-algorithms.pdf by @ctjoreilly in commit 80286be. Edited the link to reflect that change and point to a valid URL.

this classifier of learning module is not optimized to run on this huge MNIST data

having this flag drastically reduces time to load & sanity check large image datasets for practical ML tasks

* 'master' of https://github.com/aimacode/aima-python:
  Fix Description mismatch with code.