irssinotifier.pl: move sending to its own thread

Even with the wget timeouts irssi will still block for a while
during network outages. Move sending logic inside its own thread
to avoid this.
irssinotifier.pl: add queuing to send_notification

Queue up to 10 notifications if sender thread is busy. Won't send
messages over 60 seconds old.
Merge pull request #122 from qvr/threaded

irssinotifier.pl: move sending to its own thread