Periodic watcher operation modes

EvPeriodic watcher works in different modes depending on the offset , interval and reschedule_cb parameters.

Absolute timer . In this mode interval = 0 , reschedule_cb = null. This time simply fires at the wallclock time offset and doesn't repeat. It will not adjust when a time jump occurs, that is, if it is to be run at January 1st 2014 then it will run when the system time reaches or surpasses this time.
Repeating interval timer . In this mode interval > 0 , reschedule_cb = null; the watcher will always be scheduled to timeout at the next offset + N * interval time(for some integer N ) and then repeat, regardless of any time jumps.

This can be used to create timers that do not drift with respect to system time:
```
<?php
$hourly = EvPeriodic(0, 3600, NULL, function () {
  echo "once per hour\n";
});
?>
```
That doesn't mean there will always be 3600 seconds in between triggers, but only that the callback will be called when the system time shows a full hour( UTC ).

EvPeriodic will try to run the callback in this mode at the next possible time where time = offset ( mod interval ), regardless of any time jumps.
Manual reschedule mode . In this mode reschedule_cb is a callable .

interval and offset are both being ignored. Instead, each time the periodic watcher gets scheduled, the reschedule callback ( reschedule_cb ) will be called with the watcher as first, and the current time as second argument.

This callback must not stop or destroy this or any other periodic watchers, ever, and must not call any event loop functions or methods. To stop it return 1e30 and stop it afterwards. An EvPrepare watcher may be used for this task.

It must return the next time to trigger, based on the passed time value (that is, the lowest time value larger than or equal to the second argument). It will usually be called just before the callback will be triggered, but might be called at other times, too.
Example #1 Using reschedule callback
```
<?php
// Tick each 10.5 seconds

function reschedule_cb ($watcher, $now) {
   return $now + (10.5. - fmod($now, 10.5));
}

$w = new EvPeriodic(0., 0., "reschedule_cb", function ($w, $revents) {
   echo time(), PHP_EOL;
});

Ev::run();
?>
```