Add nickname auto-completion feature

Those changes extend auto-completion with nicknames :
* is_buddy() is modified to return the effective jid even if we pass a nickname
* Readline can add only one character (space) when the word is auto-completed, so I decided to add ':' in the list of complete matches, ':' character will be also a separator to parse and locate the jid/nickname
* The only problem here is that the command won't be executed if there is one space before '/' (well, like irssi..)

The purpose of this request is to satisfy users which prefer nicknames instead of jid, I am here talking about regular xmpp servers and not fb