reduced verbose on showing time stamp on history view

Fixed issue #273
We don't want to compare against last dates *in general*, we want to compare against last dates within the same list of versions.  If you keep this as a global, then you will end up sometimes showing dates and times for top-level notebooks and sometimes not:
![image](https://cloud.githubusercontent.com/assets/1366709/4208699/6ddfa4ec-3862-11e4-8346-56c000407751.png)
(Notice that there's one notebook which is showing "..." for a date, which is pretty weird too.)