Improve notifications drop-down menu styling in header

Marshalrusty: "The 3.1 notifications system is a leap in the right direction, but the contents displayed are very difficult to discern "at a glance".

I don't have the perfect solution for how to do this, but the key is brevity. I'd do something like this"

http://area51.phpbb.com/phpBB/viewtopic.php?f=81&t=45657

Eventually we might want to redesign the whole contents of the drop-down, but for 3.1, we should at least have a somewhat better system that's easier/faster to read.