When a user is idle, display the time since they were last active

I'm sending this through for a peer review of this functionality.

The time displayed is affected by issue #688 as it is displaying difference between local time and UTC time.

![idle-since-display](https://f.cloud.github.com/assets/1035315/299719/30525356-9582-11e2-859d-0a8dff472bb6.png)