Track extension usage

[rstuven]

The objective is to design new features taking into account current usage patterns. Privacy is a concern, so we'll only track interaction events (eg. button clicks) and general statistics (eg. number of terms, min/max/avg term length), not actual filter terms data.

Google Analytics seems a good way to go: http://developer.chrome.com/extensions/tut_analytics.html
See also https://developers.google.com/analytics/devguides/collection/gajs/eventTrackerGuide