title: How to get the 'n' weekday of a Date
body: <p>I need to translate this function into swift.
  Basically what does it get the 'n' day of the current week. So for example if i use it with NSDate().getWeekDay(0) it gives me Sun 11 Sept, and so on.
  But seems rangeOfUnit no longer exists in Swift-3.
  This was my previous implementation in Swift-2</p>
  
  <pre>
  extension NSDate {
      func getWeekDay(day: Int) -> NSDate {
          var beginningOfWeek: NSDate?
          NSCalendar.currentCalendar().rangeOfUnit(NSCalendarUnit.WeekOfYear, startDate: &beginningOfWeek, interval: nil, forDate: self)
          let comps = NSDateComponents()
          comps.day = day
          comps.minute = NSCalendar.currentCalendar().component(NSCalendarUnit.Minute, fromDate: self)
          comps.hour = NSCalendar.currentCalendar().component(NSCalendarUnit.Hour, fromDate: self)
          let nextDate = NSCalendar.currentCalendar().dateByAddingComponents(comps, toDate: beginningOfWeek!, options: .SearchBackwards)
          return nextDate!
      }
  }
  </pre>