Use async version of Google Analytics + categories event tracking

https://developers.google.com/analytics/devguides/collection/gajs/
https://developers.google.com/analytics/devguides/collection/gajs/eventTrackerGuide

Also improved tests to let Capybara wait for elements
Merge pull request #226 from codeforamerica/ga-event-tracking

Use async version of Google Analytics + categories event tracking