Track cohort in _setCustomVar at Visitor level scope

[kingo55]

Typically, most experimentation platforms will look at unique visitors and unique conversions. This is important because people may be influenced by a particular change in any one of their visit sessions to the site.

Basically, we need to update the storage adapter to capture this data:

_gaq.push(['_setCustomVar',{slot},{name},{value},{scope}],['_trackEvent'...]);

I suggest we allow for setting the {scope} and setting the {slot}. The name and the value can be pre-filled using the test parameters. Note it must be called before _trackEvent().

Full instructions are here: https://developers.google.com/analytics/devguides/collection/gajs/gaTrackingCustomVariables

[kingo55]

Note this also means we have to set cookie expiry for 2yrs or for just the session (depending on the scope we choose).