First implementation of an event scheduler API.

Scheduler works with the same event type as evqueue_t.

Example:
evsched_t *s = evsched_new();
evsched_schedule_cb(s, myfunc, data, 1000) // Schedule myfunc() after 1000ms
event_t *ev = evsched_next(); // Run event scheduler
evsched_event_free(s, ev); // Free executed event
evsched_delete(s);

Commit refs #799.