Fixes setTimeout issue that prevented end of execution.

When you created a node.js program that depended on `node-auth0`, after executing everything on the event loop the program didn't close. That was closed because of this:

```js
setTimeout(function () {
      delete self._currentAccessToken;
    }, 1000 * 60 * 3); //~ 3 hours
```

It basically expires the access token after 3 hours, which makes sense.

I'm adding an [unref](http://nodejs.org/api/timers.html#timers_unref) to the timer so it closes the program if there is nothing else happening in the event loop. As doing many `unref`s on `setTimeout`s timers may have performance issues I'm submitting this for review.