Adding integration test to test this code.

I also updated [ tracking api reference] to explain "cid" parameter.

cid — (requires token_auth to be set) defines the visitor ID for this request. You must set this value to exactly a 16 character hexadecimal string (containing only characters 01234567890abcdefABCDEF). When specified, the Visitor ID will be “enforced”. This means that if there is no recent visit with this visitor ID, a new one will be created. If a visit is found in the last 30 minutes with your specified Visitor Id, then the new action will be recorded to this existing visit.