Adds integration_test.go with four test functions that exercise
all major IRC features using real TCP connections:
TestIntegrationTwoClients: sequential two-client test covering
NICK/USER registration, JOIN, PRIVMSG (channel + DM), NOTICE
(channel + DM), TOPIC (set/get/lock/unlock), MODE (query, +m,
+v, -t/+t), NAMES, LIST, WHO, WHOIS (with channels), LUSERS,
NICK change (with relay), duplicate NICK, KICK (with relay +
reason), KICK non-op error, PING/PONG, unknown command, MOTD,
AWAY (set/clear/RPL_AWAY on DM), PASS post-registration, PART
(with reason + relay), PART non-existent channel, user MODE
query, multi-channel messaging, and QUIT (with relay).
TestIntegrationModeSecret: verifies +s mode can be set and
is reflected in MODE queries.
TestIntegrationModeModerated: verifies +m blocks non-voiced
users and +v enables sending in moderated channels.
TestIntegrationThirdClientObserver: verifies three-client
channel message fanout.
closes#97
