Add dart-ajax

Split out core-ajax and core-xhr from the previous PR.

I addressed some of Siggi's comments from the other PR. The others had to do with the state of the code that I ported, style and specific APIs. I tried to make as few changes as possible, so I didn't want to do those now to make tracking easier.

The demo works, but still requires that extra template for some reason.
+1, I think something like this would work:

    DartCoreAjax ajax = querySelector('dart-ajax');
    ajax.onCoreResponse.listen((event) {