Add support for HTMLTemplateElement

For browsers that has native support for template elements we just need to
ensure that content is wrapped. For other browsers we move the template content
into a document fragment as required by the template spec.
Merge pull request #34 from arv/template-element

Add support for HTMLTemplateElement
Implement the event retargetting algorithms for adjusted parent and retargetting

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