Allow ip and optional_params arguments to be accepted by all Mixpanel::People methods (except #destroy_user)

Motivation: allow the unsetting of a people property without modifying the person's "Last Seen" time, e.g.:

```Ruby
tracker = Mixpanel::Tracker.new
tracker.people.unset("12345", "Overdue Since", nil, "$ignore_time" => true)
```