Update Canadian holidays

* Add Nova Scotia Heritage Day, falling on the third Monday of
  February from 2015.
  (https://en.wikipedia.org/wiki/Islander_Day#Nova_Scotia_Heritage_Day)

* Add Islander Day for Prince Edward Island, falling on the third
  Monday of February.
  (https://en.wikipedia.org/wiki/Islander_Day#Islander_Day_.28Prince_Edward_Island.29)

* Add Northwest Territories, Nunavut, and Prince Edward Island to the
  list of regions to celebrate the first Monday of August as Civic
  Holiday.
  (https://en.wikipedia.org/wiki/Civic_Holiday)

* Add New Brunswick Day, falling on the first Monday of August.
  (https://en.wikipedia.org/wiki/Civic_Holiday)

* Add missing tests for the first Monday of August.