Finish event dispatching

This implementes stopPropagation, stopImmediatePropagation and fixes issues
with AT_TARGET.
Rename .node to .impl since these are not always nodes.
Add a method to reset the node wrapper internal pointers.
Implement the adjust related target algorithm

https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/shadow/index.html#dfn-adjusted-related-target
Merge pull request #39 from arv/rename-node-to-impl

Rename .node to .impl since these are not always nodes.
Merge pull request #40 from arv/update-node-pointers

Add a method to reset the node wrapper internal pointers.
Merge branch 'stop-propagation' of https://github.com/arv/ShadowDOM into arv-stop-propagation

Conflicts:
	src/wrappers/Event.js
Merge pull request #41 from arv/retarget-related-target

Retarget related target
Fix .node -> .impl rename in EventTarget

Out of order merges led to a badly fixed conflict