Fix for count of tasks update value & generating simple id for new tasks

Only two cases, not valid reason for creating a watcher, so this was my
approach. To modify the $scope.cont variable whenever it changes, adding or deleting a task/tasks
(Anyway, that's outside of the scope of the PR. I think the only problem was using assignment AND the decrement/increment operators. I was just answering to a more-general design question.)