Add union

Ok, for real this time? @ttrefren @tdumitrescu 
so a call like `people.union('mykey', 'not an array')` will debug-log "MIXPANEL PEOPLE REQUEST (QUEUED, PENDING IDENTIFY)" but not actually do anything, right? I'm not thrilled by silent failures of this sort, especially when there's a chance to alert developers here that they're doing it wrong.