Don't flush events on set

Hi,

We have a use case to not flush the events.  

Take for example, the code in [issue #13](https://github.com/uzikilon/backbone-poller/issues/13):

```javascript
var poller = Backbone.Poller.get(model, { delay: 1000 });
poller.on('success', function(){
  if(condition){ 
    poller.set({delay: 2000}).start(); 
  } 
});
poller.start();
```

Once the `set` method is called, `poller` will no longer have a success callback.  This seems counter-intuitive.  The only way to have `poller` always have an on-success callback is to pass it in as an option.  This is non-ideal, as I would have to pass in the options every time I want to change the `delay`.

I think it is fine to remove `this.off()`, as long as the same callback is not bound multiple times.

Thanks again,

Ken