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.

src/irc/irc_channel.cpp

@@ -0,0 +1,24 @@
+#include <irc/irc_channel.hpp>
+#include <utils/make_unique.hpp>
+
+IrcChannel::IrcChannel():
+  joined(false),
+  self(nullptr)
+{
+}
+
+void IrcChannel::set_self(const std::string& name)
+{
+  this->self = std::make_unique<IrcUser>(name);
+}
+
+IrcUser* IrcChannel::add_user(const std::string& name)
+{
+  this->users.emplace_back(std::make_unique<IrcUser>(name));
+  return this->users.back().get();
+}
+
+IrcUser* IrcChannel::get_self() const
+{
+  return this->self.get();
+}