Scheduler integration

Hi Vincent,

I think the scheduler integration is almost ready so please take a look. As we previously discussed, I've also added two new methods ``schedule_at`` and ``schedule_in`` to ``Queue``.

As usual, comments are welcome.

Next up, I plan to work on a feature so we can check whether a callable is already scheduled for execution (``if func in scheduler``). If this goes well, we can do something similar for queued jobs as well (``if func in queue``).