Change the data struture feature_item and modify the add_feature function
Modify the structure of SQL_Parser, make it more simple and the detail is left to the derived class
Modify the algorithm of parse, add some functions to deal aggregate function in SQL
split the big SQL_Parser.cc into threee part according to its function and fix some bugs
Implement function to handle between...and..., fix some bugs in get_word function
Merge branch 'testing'