[user-analytics] Added support for Google Analytics.

RELATED ISSUE : #956
RELEASE NOTES :
This commit adds support for the "Universal Analytics" flavor of GA (analytics.js).
Concerto respects the "Do-Not-Track" setting set by the user-agent.
I kept it a separate initializer to make it easier to remove out for any users that don't want Google Analytics support.

Also, I plan on refactoring it into a plugin (very soon) when I figure out better integration points in Concerto proper.