Re: [fetch] Aborting a fetch (#27)

@jan-ivar 

> @benjamingr My library has no inherent interest in absorbing responsibility over whether what came before should be cancellable or not

I think I might have miscommunicated. Your example absolutely does not leak a reference with refcount semantics. My note was with regards to unhandled exception tracking and its relation with cancellation. See https://jsfiddle.net/v6LrLdsj/ 

```js
var p = cancellablePromiseFactory();
p.catch(() => {});
p.cancel(); // p gets cancelled here
```

On the other hand:

```js
var p = cancellablePromiseFactory();
var p2 = p.then(...);
var p3 = p.then(...);
p2.cancel(); // refcounting semantics means it's unsound to abort here
```

Example: https://jsfiddle.net/1vfq7oq1/

I assumed you tried these semantics out first before you engaged in this debate. Feel free to use the fiddle links as a playground for testing out bluebird's cancellation. 

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