Creates mapping key = word, value = row index

def _create_dictionary(self, document):
        """Creates mapping key = word, value = row index"""
        words = map(self.normalize_word, document.words)
        unique_words = frozenset(self.stem_word(w) for w in words if w not in self._stop_words)

        return dict((w, i) for i, w in enumerate(unique_words))