Returning the relative time in seconds, instead of nanoseconds, in the method `_now`.

The unit is clearly documented by Apple in QA1398:
https://developer.apple.com/library/mac/#qa/qa1398/_index.html. I also
verified the unit "manually" by comparing elapsed times measured with
NSDate and with `_now`.
Adjusting timer delay to take into account the time elapsed between the call to `afterDelay:do:` and the execution of the block that actually schedules the timer.