Handle bubbling event properly

This is fix for link_attr problem.

For example:
```django
<a href="#" {{ link_attr('delete', "my confirm message") }}>
  <img src="http://rad.knplabs.com/images/knp_logo.png" />
</a>
```

In this case, `event.target` is not `<a>` element but `<img>` element. So we'll get default confirm message, "Are you sure?".
So I think you should use `event.currentTarget` instead of `event.target`.