off
Remove an event listener.
    compute.off(eventType, handler)
  
  var age = compute(33);
var handler = function(ev, newVal, oldVal){
    ev //-> {type: "change", batchNum: 5}
    newVal //-> 34
    oldVal //-> 33
}
age.on('change', handler);
age(34);
age.off('change', handler)
  
  Parameters
- eventType 
{String}:The name of the event to bind on, usually
change. - handler 
{function(event, args...)}:The handler to be removed. This has to be the same function that was passed to on.