Add example for declarative event handlers recommendation

[addyosmani]

Per: https://twitter.com/WebReflection/status/506417714454597634

We should add more information on why we suggest using declarative event handlers rather than addEventListener. We cover a little in http://www.polymer-project.org/docs/polymer/polymer.html#declarative-event-mapping, but more about how to handle say, on-google-data-success etc. would be useful.