notify module depends on GNU date

`%N` is only available in GNU date:
- changed the notify module to use seconds instead of nanoseconds
- changed format-elapsed to not print *x.0 secs* but just *x secs* (just my preference, left in a separate commit so it can be removed)

I also have a function that uses AppleScript to only send the notification if the frontmost window when the command finishes is not the same one that started it, but it needs some polishing, will send in a separate pull request if interested.