Update event.relatedTarget as needed.

This also contains fixes for the following:

1. Allow listening to event on the shadow root by addind the real event listener
   to the host.
2. If, after retargetting, we end up with target === currentTarget we need to
   change the phase during capture and ignore during bubbling.
Merge pull request #49 from arv/related-target

Update event.relatedTarget as needed.