FEATURE: Mark All as Read button for Notifications page

Added a Mark All as Read button to the top/bottom of the notifications user page
https://meta.discourse.org/t/possibility-to-selectively-or-completely-mark-notifications-as-read/20227

![image](https://cloud.githubusercontent.com/assets/2060315/4612152/a774efa2-52c3-11e4-94ca-bda9abdfa34b.png)