Improve Private Storage detection. MCabber now correctly detects Private Storage support when receiving an item-not-found error (which doesn't mean the server has no support for P.S., but that it's empty). I've added a return value to the callback functions. For now the return value is only used for IQ error handling.

 /SAY_TO jid text

Send the "text" message to the specified jid.
Please note that this command doesn't set the default resource for a contact, so if you want to send several messages to a specific resource you will have to use "/say_to" for each message.