Add "previousMonth" and "nextMonth" function to object "YMD", and "previous" and "next" constants to class "SimpleCalendar"
Refactoring:
* Add the validation to classes "Month" and "Day".
* Add "isFirst" and "isLast" methods to class "Loop".
* Add the constant "min" to class "LoopCompanion".
* Add "nonSunday" and "nonMonday" methods to class "Day".
Modify the exception message of object "LoopCompanion".
Change the view.
Change the view for class Head.