Some idiomatic improvements

Minor improvements to fit in with how we generally build Polymer elements.

Adding some other notes in case you have time before your talk:

* Across all implementations I would probably add a loading interstitial (text or gif) to allow the audience to know you're waiting for contacts data to load. I initially opened most pages expecting to see the data pretty quickly after, but (maybe due to Heroku) it took much longer than expected. A simple show by default / hide on request complete could cover you for your talk.

*  Most of us on the Polymer team use https://github.com/Polymer/core-ajax for requesting data rather than writing our a whole XHR handler. Depending on how long you have before your talk, it might be worth looking at it. You'd include it as an import and then consume. Docs are at http://polymer.github.io/core-ajax/components/core-ajax/


GL with the talk!