Get the nth weekday in a given month. e.g:

        >>> # the 1st monday in Jan 2013
        >>> Calendar.get_nth_weekday_in_month(2013, 1, MON)
        datetime.date(2013, 1, 7)
        >>> # The 2nd monday in Jan 2013
        >>> Calendar.get_nth_weekday_in_month(2013, 1, MON, 2)
        datetime.date(2013, 1, 14)

def get_nth_weekday_in_month(year, month, weekday, n=1, start=None):
        """Get the nth weekday in a given month. e.g:

        >>> # the 1st monday in Jan 2013
        >>> Calendar.get_nth_weekday_in_month(2013, 1, MON)
        datetime.date(2013, 1, 7)
        >>> # The 2nd monday in Jan 2013
        >>> Calendar.get_nth_weekday_in_month(2013, 1, MON, 2)
        datetime.date(2013, 1, 14)
        """
        # If start is `None` or Falsy, no need to check and clean
        if start:
            start = cleaned_date(start)

        day = date(year, month, 1)
        if start:
            day = start
        counter = 0
        while True:
            if day.month != month:
                # Don't forget to break if "n" is too big
                return None
            if day.weekday() == weekday:
                counter += 1
            if counter == n:
                break
            day = day + timedelta(days=1)
        return day