Export State.OnInterface (#417)

event.go

@@ -224,7 +224,7 @@ func (s *Session) onInterface(i interface{}) {
 	case *VoiceStateUpdate:
 		go s.onVoiceStateUpdate(t)
 	}
-	err := s.State.onInterface(s, i)
+	err := s.State.OnInterface(s, i)
 	if err != nil {
 		s.log(LogDebug, "error dispatching internal event, %s", err)
 	}

state.go

@@ -783,7 +783,7 @@ func (s *State) onReady(se *Session, r *Ready) (err error) {
 }
 
 // onInterface handles all events related to states.
-func (s *State) onInterface(se *Session, i interface{}) (err error) {
+func (s *State) OnInterface(se *Session, i interface{}) (err error) {
 	if s == nil {
 		return ErrNilState
 	}