Preserve context

I just started with bacon.js and tried to get a stream from an input field:

```javascript
var foo = $("#bar").asEventStream("change", function() {
    return $(this).val();
});
```

But it did not work because ```this``` references ```window```. I think it should reference the DOM input element. Am i using bacon.js in a wrong way?
If not, this pull request fixes it.