XMPP - added multi user chat feature and corresponding scripts and rules action

Added the possibility to join a multi user chat (aka chat room) and added a new method 'chatXMPP' to send messages to the multi user chat.

Now your house can chat the whole day long what's happening and alle members of the chat are up to date. 
It's some kind of 'Twitter' gone XMPP :-)

If desired I could write a little wiki page on how to set up your own private jabber server.