Support querying the worker for the currently executing job.

Adds the possibility for pipelining (similar to Job.save and Job.delete) by adding optional pipeline=None parameters. Upon starting a job, right after setting the 'busy' state, saves the id of the job to be executed. A convenience method for the probably most common use case is also available (``Worker.get_current_job()``), which returns a ``Job`` instance.

I did not implement pipelining at this time, mainly because I am not 100% of the implications. It would also require reordering of the following code:

    self.state = 'busy'
    job, queue = result

Someone who knows the codebase better could easily add it though.