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.
@benogle I think the pattern should be this:

```coffee
createdCallback: ->
  @appendChild(Content.cloneNode(true)) # Clone/append the entire document fragment
  @rootTemplate = @firstChild # We know our first child is a template in this case

```