returns tf-idf matrix
       (unigrams+bigrams)

def _normalize(sentences):
    '''returns tf-idf matrix
       (unigrams+bigrams)'''

    tfidf = TfidfVectorizer(tokenizer=_tokenize,
                            stop_words='english',
                            decode_error='ignore',
                            ngram_range=(1,2))
    return tfidf.fit_transform(sentences)