Improved on Singly Linked List Programs (#1558)

* Improved Singly Linked List
Added String Representations of Nodes and Linked Lists
Added support for indexing and changing of Node data using indices.

* Added a few comments to Linked Lists

* Reformatted to conform to PEP8

* Added from_sequence.py
Convert a Python List to Linked List comprising of Nodes and return head.

* Added print_reverse.py
Recursive program to print the elements of a Linked List in reverse.

* Change 'is not None' for more Pythonicness