Added GA event for starting an interactive

This will trigger an GA custom event when starting an interactive. The event that are fired can be seen here:
https://www.google.com/analytics/web/?hl=en&pli=1#realtime/rt-event/a6899787w68675665p70722550/

The PT story is:
https://www.pivotaltracker.com/projects/731349#!/stories/46994677