Refactor global gaTrack function. Bug 896001.

- Takes a single array of event data (instead of an array of arrays).

- Only sets hitCallback/setTimeout if a callback exists.

- Existing newsletter JS only sets callback for last event track.

- Existing newsletter JS tracks both new subscriptions and unsubscriptions.
Merge pull request #1104 from mozilla/bug-896001-newsletter-ga-refactor

Refactor global gaTrack function. Bug 896001.