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.
wonder if all these helps could be in a separate file, and your element can load it?

<script>...</script>
<script src="ga_helpers.js"></script>