1. Moved out the logic in private instance method
		validate_given_week_number_in_february_given_year

	in Calendar class to a class method having signature

		validate_given_week_number_in_february_given_year(year, month, week_number)

	in module ClassMethods.

2. Added following class methods in module ClassMethods
	start_date_of_week_number_in_month(week_number)
	starting_weekday_of_week_number_in_month(year, month, week_number)

3. Updated implementation in following class methods in module ClassMethods

		week_number_of_month(time=Time.now)
		week_number_of_year(time=Time.now)

	by adding a check to raise an error if the argument received is not of
	type Time.