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.
you may want to do these conversions in a changed handler:

    startDateChanged() {
       this.startDate = convertDate(this.startDate);
    }

This won't get you in an infinite loop because `startDate` will eventually stop changing and the system won't call this function.