Only reconnect to IRC if not shutting down.

As things stand right now, when the irc bot loses its connection, it
just tries to reconnect.  If it can't do that, then it waits 60 seconds
and tries again.. forever.  This is good, generally.  If our bot goes
offline due to a netsplit or something, it can get itself back on
freenode without human intervention.

However, if we're trying to stop the bot.. say, with
``service fedmsg-hub restart`` or a simple control-c, then it loses its
connection and, before it can finish terminating, tries to reconnect
again.  It never actually exits and you have to hit it with ``kill -9``.

This commit makes it so that when the core hub calls ``.stop()`` on the
``FMNConsumer``, then that consumer will tell its subordinates to give
up the :ghost:.
Merge pull request #19 from fedora-infra/feature/careful-with-the-irc-reconnects

Only reconnect to IRC if not shutting down.