Switch to Google Analytics API

This removes all of our Webtrends API code with Google Analytics API code.

It adds two new dependencies to vendor in `vendor/packages` and a new compiled dependency (PyOpenSSL). So `pip install -r requirements/compiled.txt` or equivalent is required to run the new code. We are blocked on IT installing this on our servers in order to land this.

The API calls are mocked for testing. To run the real thing, ping me for the authentication settings required. The three cron jobs that depend on this are:

```
$ ./manage.py cron update_visitors_metric
$ ./manage.py cron update_l10n_metric
$ ./manage.py cron reload_wiki_traffic_stats
```

That last one takes a while. The first two update data that you can see on the KPI dashboard. The last one updates data you can see on the KB dashboards (`/en-US/contributors` and `/es/localization`).

That's all I can think of right now. r?