Modified shebang of spell_checker to use python2 explicitly since nltk does not support python3
Removed the memoize import statement in spell_checker since memoize is now defined in spell_checker.
Modified authorship of spellcheck.py to reflect appropriate author.
Created sp_prop.py
Moved edit_distance & memoize to sp_prob.py
Renamed spellcheck to sp_correct.py
Refactored edit_distance to a class EditDistance.