Initialize crontab module, called by engine.

Register a callack which will be executed when time condition is satisfied param minute: time condition satisfied on the specified minute, or every -minute if minute is negative param hour: time condition satisfied on the specified hour, or every -hour when hour is negative param day: time condition satisfied on the specified day, or every -day when day is negative param month: time condition satisfied on the specified month, or every -month when month is negative param dayofweek: time condition satisfied on the specified week day, or every -dayofweek when dayofweek is negative param cb: callback function to be executed when time is satisfied.

Handle is the type of return value of Register, can be used to cancel the register.