issue 22: advertise/broadcast message

hi @whimboo !

this pull request includes the issue 28 updates. 
the updates particular to issue 22 are those whose commit message begins issue 22.
I'm not sure how best to test this. Is it ok to create a few test channels? But this code is also in fennecbot, hardly changed.
And perhaps you'd prefer I resubmit after you merge issue 28 updates?


We don't reset this message currently, which should IMO be done when the testday ends. Also we should only call `client.say()` if the message is valid.