Save last activity time

Dependent on #995, solves #997.

Before, UserLogSummary objects just kept track of summary stats within some time range.  Now, they also keep track of the last user activity timestamp within that window.

![image](https://f.cloud.github.com/assets/4072455/1507416/98f3dc70-497f-11e3-8331-7e6230105836.png)