finished the draft of new util
removed unnecessary upper
fixed the typo in a comment
fixed the typo in the list of allowed operators
let user customize boolstyle by overriding stringify_bool
enabled the feature of delimiting identifier
added some tests
raise assert error if the op is not allowed
make flow better
make prefix upper when init
use simpler procedure
added __all__
support to close the feature of delimiting identifier
support to close the feature of checking the operator
renamed aggregater
fixup! fbdd833
changed the order
added the first docs
rewrited mosql/mysql.py
added statement of doctest