Display the 'Last Seen' value on the Users listing screen

This would be a nice addition, but it does add clutter to the Users listing screen. Feedback welcome.

![screen shot 2014-10-14 at 13 32 35](https://cloud.githubusercontent.com/assets/208434/4628947/3c24ab12-539e-11e4-899d-cb73e05e2d12.png)