TomeRater

I have added documentation inline within the TomeRater explaining changes made to support unique ISBN functionality. These changes will cause populate.py, as originally written, to throw an error since the Book.set_isbn() method has been relocated to the TomeRater class. The hash method in Book was broken as it was based on a mutable attribute, isbn. Updating the ISBN directly on the Book object caused duplicate entries in the TomeRater.books dict since the hashes were different pre and post ISBN update, despite the contents being the same. The Book object is now removed from the dict, ISBN updated, and then the Book is placed back into the dict with the new hash in tow.