Completed issue #19. Allow gtfs.getStopsByRoute() to return all stops for all directions if direction_id is not specified (direction_id = null).

Updated test cases to cover new behaviour.
Merge branch 'develop'

* develop:
  Completed issue #19. Allow gtfs.getStopsByRoute() to return all stops for all directions if direction_id is not specified (direction_id = null).
Merge pull request #20 from sogko/master

Allow gtfs.getStopsByRoute() to return all stops for all directions if direction_id is not specified (direction_id = null)