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.
yeah, it's a copy of the polymer JS docs. If we had infinite time, we would rewrite them all into Dart style. Unfortunately we're very pressed for time at the moment :)

At this point filing a bug is the most helpful way to remember these, so I opened:
https://github.com/dart-lang/core-elements/issues/4