seconds granularity in timesince(), fixes #45

@twobraids r?

This is really quite unimportant. I just wanted to be able to have something like "3 seconds ago" (instead of "0 minutes ago") when you run `--list-jobs` and it notices that a job started, say, 3 seconds ago. 

The `timesince()` function is only ever used in the `--list-jobs` switch which isn't very operationally important.