weekOfYear与
yearForWeekOfYear以下产品配合使用：
let calendar = NSCalendar.currentCalendar()let components = NSDateComponents()components.yearForWeekOfYear = 2015components.weekOfYear = 15print(calendar.dateFromComponents(components)!)// 2015-04-05 22:00:00 +0000
更新 斯威夫特3 及更高版本：
let calendar = Calendar.currentvar components = DateComponents()components.yearForWeekOfYear = 2015components.weekOfYear = 15print(calendar.date(from: components)!)
