Add ability to reschedule onto the current scheduler

[lewissbaker]

Add overloads of the schedule(), schedule_after() algorithms that don't take a scheduler and that use whatever the current scheduler from the receiver is (ie. using get_scheduler() - see #20).

This would simplify writing some kinds of code, like a timeout() algorithm.

template<typename Op, typename Duration>
auto timeout(Op op, Duration duration) {
  return take_until(std::move(op), schedule_after(duration));
}

Whereas currently we'd need to do something like:

template<typename Op, typename Scheduler, typename Duration>
auto timeout(Op op, Scheduler sched, Duration duration) {
  return take_until(std::move(op), schedule_after(sched, duration));
}

Q. Should schedule() be named reschedule()?