Job registry to cheaply determine whether a callable is scheduled

Hi Vincent,

This pull adds the capability to check whether a callable is scheduled for execution. I implemented this using a ``Registry`` class (naming suggestions welcome) that maps functions to job ids.

What I like about this approach is that it can be very cheaply done so we can implement something for ``Queue``, which may contain a much larger number of jobs.

Furthermore, this also allow us to build much better monitoring tools, now that we can show job statistics/distribution of a queue. I'd imagine something like this would be quite useful:

Queue        : default
Total jobs   : 15
send_mail   : 10
gen_report   : 5

Thoughts?