Workaround <use> element event path

The SVG <use> element has a horrifying "instance tree" related to ShadowDOM, but does not retarget.
The SVGElementInstance (the instanceRoot type) has parts of a Node's API, but not all (like .contains).
For native events the SVGElementInstance will bubble events up to the <use> element, even non-bubbling events like enter
and leave, but it will not do so when dispatchEvent is called on it with a non-bubbling event.
Only Safari, IE, and Chrome implement this monstrosity, and working group seems to agree it should be removed:
http://lists.w3.org/Archives/Public/www-svg/2014Jan/0014.html

This workaround rewites event.target and relatedTarget to refer to <use> elements and not their "instance trees".
Fixes #124
Merge pull request #128 from Polymer/svg-use-target

Workaround <use> element event path