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
Increment and decrement operators are actually unary. This means that they already modify the value of the variable. Assigning the result is prone to error because it depends whether it is post or pre increment/decrement.