Add first draft of rot13 assignment.

Add the file "rot13.py". The module is complete but it does not run.
It is broken. Also there is little formatting or commenting.
Make changes to file.

Alter the source code to make the file run.
The module raises at least one AssertionError.
Add final version of rot13.py

This is the final version of "rot13.py". The module runs and has formatting
with comments to explain the components. "All Tests Pass" displays in the interpreter
to verify that it is fully functioning.