Support new operations in Arithmetic-Example (append, remove/delete)

New operations can be supported in Arithmetic-Example to provide a more succinct demo for the future users/developers.

1. Adding "append" 

        Command: ./client.sh append --name X --value Y
        Behavior: This will increment the variable X by Y.
        Alternative: Improve "assign" to do variable increments
                             ./client.sh assign --name X --value X+Y
                             [This throws an Exception today]

2. Adding "remove/delete" (let's just say remove for now)

        Command: ./client.sh remove --name X 
        Behavior: This should remove variable X from the cluster. 
        Alternative: None