Added support to send userId and userInfo while authenticating

In the current lib, when authenticating it is not possible to add userdata to the request. 

It is possible add some headers in:

```objective-c
- (void)pusher:(PTPusher *)pusher willAuthorizeChannelWithRequest:(NSMutableURLRequest *)request
```

but it is hard to alter the post var's there. 

This commit allows you to set the userInfo and userId so the authentication script can use it in the presence_auth method:

e.g:

```php
echo $pusher->presence_auth($_POST['channel_name'], $_POST['socket_id'],($_POST['user_id'], ($_POST['user_info']);
```