Hook startup to 001 RPL_WELCOME.

Related to issue #348 and might fix it.
Add Bot.connection_registered and re-add rfc1459 connections.

Listen for the rfc1459 connection registration message in case there exists
a server somewhere that sends that message but not the rfc2812 RPL_WELCOME.
[coretasks] Fix KeyError on QUIT

The trigger object passed by a QUIT event doesn't include a sending
channel, so
bot.privileges[trigger.sender] in coretasks.track_quit() was throwing a
KeyError. The bot now
loops through and removes the triggering nick from privileges for all
channels, avoiding the error.