Add unread reply count to header for in-app visual notifications of replies

One of the things that bothers me most about HN is the lack of any good visual notification of new replies to comments. While pushover and email notifications are great options, some users may prefer a less intrusive manner of reply notifications. This commit adds an unread reply count in parentheses next to the "Your Threads" link - the unread reply count resets when "Your Threads" is visited and looks like this:

![screenshot](http://i.imgur.com/nOCSO.png)