Add 'eid' to be generated for each event

- Initially, we added the eid to Subject class. When we created a
  tracking event it would only add an eid if you created a Subject.
  Along with that, the eid is only generated once and then reused in
  several events sent - we do not want this. Generating the eid in
  `completePayload` solves both of these issues for us.
- Fixes #95