Added support for visitor id retrieval and event triggering

Clients can use our JS SDK in order to identify the SPiD user before
doing any event tracking on their side. If NO SPiD user is found then
the SDK will return a unique visitor_id that can be used to track
events until SPiD can identify the user after a registration/login and
connect that visitor to a SPiD user. Upon identifying the user once,
this unique ID will always be returned.