Add time evaluation
update, use property(max_word_length) of trie
update 2.0, add new methods(for doc_similarity)

add word_count property, update properties, add merge_tf() method, add
fill_max_trie() method to extract top_n trie.
add doc_similarity

push doc_similarity, calculate target_doc_tf*idf value, pick
keywords(top 5)