show favicon notifications for unread chat messages

Firefox:
![screen shot 2013-12-17 at 8 55 55 pm](https://f.cloud.github.com/assets/1302888/1762386/72066246-66f2-11e3-894b-ce85c4e99eda.png)

Unfortunately Chrome half hides it with the camera record icon:
![screen shot 2013-12-17 at 8 56 36 pm](https://f.cloud.github.com/assets/1302888/1762392/9777977a-66f2-11e3-8c8c-fc21780e7ebe.png)

Uses the [Page Visibility API](https://developer.mozilla.org/en-US/docs/Web/Guide/User_experience/Using_the_Page_Visibility_API) and [favico.js](http://lab.ejci.net/favico.js/).

Worth adding?