Element manager spike

So, moving away from the inheritance model, I came up with an `ElementManager`. It feels pretty good, and should be easy to use from JS as well as coffee. 

@nathansobo, would love a review + ideas. It's obviously a WIP as things will need to move around. Just trying to get a handle on what feels good for the general API.
About the div. HTML imports make some part of the dom invisible for a time. Enough that measurement fails pretty hard. So this works for now. It's really the same thing, just not as clean. If they put a script tag in there, it'll get run, which is annoying, but that would happen with an import also. I cant find a way to programmatically insert string html content into a template either, as that would be next best thing.