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 the 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.
Merge pull request #6 from cparnot/master

Fix time unit for the `_now` method, and better calculation of the timer delay.