Reduce random twitter duplicates.

Changed the twitter randomizer to create a list of indexes corresponding to each possible tweet in random order.  Then tweets will be selected from that list and their index removed from the list.  Once the list is empty a new list will be generated.  This will ensure that tweets cannot be seen again until every other tweet has been displayed.  This should make it feel more random to the user even though it isn't.

Change-Id: Iedf3863d04544e3afab7e7e68d92ee857df3d063