PHP login abstraction. (include/login.php)

Check if user really exists in the database whenever login status is
checked.
Login::check will directly die status : "not_login" or "not_admin", all
the PHPs that use it need changing.
Users management JS (forget to include in commit).
All PHP respond in JSON format.

Always exist "status" and is "success" if success.
if not success, msg may be used.