NotificationService
Changed ArraryList to HashSet so that a client cannot be subscribed more than once.

NotificationServiceTest
Added numberTimesSubscribed and numberTimesSubscribed

Signed-off-by: bhill2 <bjsrhill@gmail.com>
Added numberTimesSubscribed and numberTimesSubscribed

Signed-off-by: bhill2 <bjsrhill@gmail.com>