Visitor knows when someone hear him

Some code has been copy / pasted. So even if you're interested in this feature you may want to wait for some cleaning before pulling it.
However I'm creating this pull request now in case you already have some feedback.

I believe I could add a few commits to this pull request to:
* Add timestamps to message (because it's nice to see that someone is coming (or ringing), but it's nice to see if it's just now or if it happened an hour ago)
* Send feedback (like "the bell is ringing" to the client, when he clicked, to let him know if no one heard because of his 3G)
* Remove code duplication