Remove logbook and replace it with logging.

This reverts commit 1ab8c19696cb6b437729401640d00fd682e96b27 and
reintroduces all changes made by @dstufft.

Still, it needs more patches to reeanble the default log-to-console
behaviour.  See #121.
Merge branch 'master' into remove-logbook
Only configure logging handlers if not already present.
Removed __main__ that was accidentally added to rqworker script.
Add release notes.
Have Worker.work() setup logging.

Basically, I don't want to enforce users to configure _any_ logging
stack when writing a basic worker, like this:

    from rq import Worker, Queue, Connection

    with Connection():
        q = Queue()
        w = Worker([q])
        w.work(burst=True)

In case you want to disable logging altogether, you can configure your
logging stack to do so.
Merge branch 'selwin-remove-logbook'

Conflicts:
	rq/utils.py
	rq/worker.py
	setup.cfg
	setup.py