Send a feedback to the visitor when it's actuall ringing

It's currently to every visitor currently connected because it was the most straightforward
to implement, and it shouldn't hurt
The host can say he's coming
Timestamp messages
Add some debug logs
Don't print "it's ringing" forever

I'm not applying this to the "I'm coming message" because it's normal that
this action last for some long minutes.
Merge pull request #3 from gturri/ack

Visitor knows when someone hear him