Handle IRC QUIT command

Implement part and join, both ways

Send and receive messages

Also correctly respond to PING with the id, escape some XML content, but not
always

Implement the Bridge class to translate between the two protocols

Add all useful classes as well: Jid, Iid, IrcChannel, IrcUser etc to
properly keep the informations about what we receive from the IRC server.

Only handle the MUC join stanza, and send the list of users in the IRC
channel to the XMPP user, and the IRC channel’s topic, for now.

Exit the poller when it handles no connection at all

Rename libirc and libxmpp to irc and xmpp

Move the basic socket implementation into the SocketHandler class

(that is, the read/write/connect/etc)
Because this code is actually common for both the IrcClient and
XmppComponent class. These two classes have to implement some higher level
callbacks (parsing the data provided in the buffers, doing stuff when the
connection is done) instead of doing the read/write/connect low level
things.

Basic IRC message parsing/sending

First step of the connection skeleton

Basic connect, socket creating, polling, recving, etc.