uncomment insert()

uncomment insert() to add word, and word.tf+1
use seg_list instead of seg_str

store splitted word into a list, add print_seg() method to format
output string
add insert_0tf() method

add insert_0tf() method, while add word into trie, ignore TF, means
every TF keep as 1.
Add traverse_BFS() method

Add traverse_BFS() method, Breadth-First search(layer traverse) tree,
yield words
Add merge() method, new MergeTypeError Exception

Add merge() method to merge other trie. if incoming trie in not
type(Trie), then raise MergeTypeError
Add code to deal with Unknown-word
Add item_list() method, to traverse out word_tf_list
Fixbug

workable version