Improve set read/unread in notifications dropdown

before:
![notifications_dropdown_before](https://f.cloud.github.com/assets/3798614/2489473/fea68c44-b166-11e3-8495-7c27ed6bd043.png)

after:
![notifications_dropdown_after](https://f.cloud.github.com/assets/3798614/2489474/056cda4c-b167-11e3-9291-9a83f2e147b8.png)
http://videobin.org/+7la/9yh.html