client.go
func (c *Client) AuthenticatePresenceChannel(params []byte, member MemberData) (response []byte, err error) {
	return c.authenticateChannel(params, &member)
}
func (c *Client) AuthenticatePresenceChannel(params []byte, member MemberData) (response []byte, err error) {
	return c.authenticateChannel(params, &member)
}
/*
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)

*/