Add trackSociaInteraction method

[alexanderdean]

See Google Analytics addSocial here: https://developers.google.com/analytics/devguides/collection/gajs/gaTrackingSocial

Use unstructured event tracking for this.

Function should be called trackSocialInteraction

[fblundun]

Alexander Dean (@alexanderdean) Yali (@yalisassoon) Should we put these new unstructured events into the the tracker core so they can be used in the Node.js Tracker too?

[alexanderdean]

Hmm... Good question. I reckon all these methods should go into core, otherwise we will end up making somewhat arbitrary event-by-event decisions... It won't do any harm to have them all in core.