Track API usage

We track analytics on the front end using Google Analytics or similar (e.g. piwik)

However, we don't track API usage in any systematic way. This would be useful to provide reports to users and also in order to manage API use.
## Proposal

One of the simplest options would be to plug into google analytics system but from the server side using ["universal analytics"](https://support.google.com/analytics/answer/2790010?hl=en&hlrm=de) - there's a [python universal library here](https://github.com/analytics-pros/universal-analytics-python).

```
# as python named-arguments
tracker.send('pageview', path = "/test", title = "Test page")

# as property dictionary
tracker.send('pageview', {
'path': "/test",
'title': "Test page"
})
```

We could also use this to track other activity e.g. certain kinds of events:

```
from UniversalAnalytics import Tracker

tracker = Tracker.create('UA-XXXXX-Y', client_id = CUSTOMER_UNIQUE_ID)
tracker.send('event', 'Subscription', 'billing')
```

Note: afaict alternatives to GA like piwik do not support the ["Measurement Protocol"](https://developers.google.com/analytics/devguides/collection/protocol/v1/)