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.
See what you're saying. It's nice to protect people from accidentally recycling the same content. But I feel weird about not giving people what they've asked for and introducing this insulating layer. Hmmm...