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.
It's arbitrary and subjective, but seems simpler and more efficient to avoid using a query. Also, if we suggest they always append the entire document fragment, it protects them in cases where there's more than just a single tag at the root of the HTML file.