code: func (c *Client) AuthenticatePresenceChannel(params []byte, member MemberData) (response []byte, err error) {
  	return c.authenticateChannel(params, &member)
  }
docs: /*
  Using presence channels is similar to private channels,
  but in order to identify a user, clients are
  sent a user_id and, optionally, custom data.
  
  In this library, one does this by passing a `pusher.MemberData` instance.
  
  	params, _ := ioutil.ReadAll(req.Body)
  
  	presenceData := pusher.MemberData{
  		UserId: "1",
  		UserInfo: map[string]string{
  			"twitter": "jamiepatel",
  		},
  	}
  
  	response, err := client.AuthenticatePresenceChannel(params, presenceData)
  
  	if err != nil {
  		panic(err)
  	}
  
  	fmt.Fprintf(res, response)
  
  */