Add Dynamic data over XHR example

[claytantor]

This is a great tutorial, thanks for making it but the XHR example for networked data on http://www.ng-newsletter.com/posts/d3-on-angular.html eludes me. I have spend considerable time trying to use the approach you proposed without success. Mostly because it seems that the watch and render lifecycle would be different for async. I have created the following gist to explain the ask.

https://gist.github.com/claytantor/ba49f799a9b37921d5e3

Having a working example with this code base would probably help others. I would fork and fix and submit a PR if I knew how to fix it.