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
I'm not sure about the scope of the project. Regardless, I'd make the count be a calculated property of the array, depending on the purpose. If it's for display purposes, then that could be displayed as the length property of the filtered array.