Implement the event retargetting algorithms for adjusted parent and reta...

...rgetting

https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/shadow/index.html#dfn-adjusted-parent
https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/shadow/index.html#event-retargeting

This is needed for https://github.com/toolkitchen/ShadowDOM/issues/33