Add google-analytics element files.

There are a few additional features I'd like to add, but I wanted to get the base stuff committed first.

In the future I think we can remove the dependency on the [Embed API](https://developers.google.com/analytics/devguides/reporting/embed/v1/) completely and just use `<google-chart>` and something like `<core-ajax>`, but for now this is working pretty well.
We usually do published properties for this type of thing. That way you can do `el.unrendered = false` in JS and the attribute is removed. Note, you'll need to publish the property in the `publish` block with `reflect: true`: http://www.polymer-project.org/docs/polymer/polymer.html#published-properties