adds goal to end of path in global_planner, makes move_base's planService search outwards

fixes #207
I've changed it to just "the final reachable goal should have been added by the global planner".
(I did mean local--we said we wanted to have the adding of the final goal be in the global planner and not in move_base, for generality.  TrajectoryPlannerROS expects it to be there, but if you write your own global and local planners, you could conceivably not want that.)