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.
We could get to that one day, but right now our codegen is not generating anything for event streams. We could start generating `onCoreResponse` like in dart:html in the future. Opened #16