added default mechanism
use squashed_cols instead of defaults
added clear
reorder code
removed classattr col_names
reordered
updated example for new method `new`, `clear` and `default`
use __init__ instead of new, new instead of default
fixed the compatibility with MySQL
updated examples
fixed a bug of append
use defaults instead of squashed_cols
reworded error msg
rename idx
reordered code
make perform more powerful
added method add
updated doc
added example of using perform
removed hash_dict