Avoid running listen() if the node is disconnected

[stonier]

listen() can sometimes be invoked after self.connection instance is destroyed. This just escapes the method if this occurs.

[andymccurdy]

Hi. Many apologies on taking so long to cleanup Publish/Subscribe in redis-py. I've recently had some time to make some big improvements and I'd love to get your feedback.

You can review the changes in the pubsub branch here: https://github.com/andymccurdy/redis-py/tree/pubsub

I've written a section in the README docs about PubSub that covers most (hopefully all!) use cases. You can find it here: https://github.com/andymccurdy/redis-py/blob/pubsub/README.rst#publish--subscribe

My current plan is to merge the pubsub branch into master and roll a new release of redis-py by the end of the week. Again, I'd love any feedback.