Fixed addition of multi-year duration to year_month.

* Added new overloads for operator+/- between year_month and duration
  that is convertible to years, so it is better candidate for operands
  that are convertible to both years and months. To preserve
  functionality, this operator is conditionally noexcept.

* Reworked year_month_day, year_month_day_last, year_month_weekday,
  and year_month_weekday_last.

* Added tests for this new functionality.

Author: tomaszkam