Add listener close function

[dmorn]

Close #26.

I'm doing graceful shutdowns by wrapping the RTMP server and adding context to the ListenAndServe function

func (s *Server) ListenAndServe(ctx context.Context) error {
	go func() {
		<-ctx.Done()
		s.rtmps.Close()
	}()
	return s.rtmps.ListenAndServe()
}

When the context is canceled, ListenAndServe will stop blocking.