Add support for connecting to Redis with a URL

These changes add support for connecting to a Redis instance using a URL (e.g. `redis://user:pass@hostname:9001`). The URL can be specified in `settings.py` as `REDIS_URL`. If `REDIS_URL` is specified, it takes precedence over the `REDIS_HOST` and `REDIS_PORT` settings.

I added a number of tests for this new functionality and also updated the README to reflect these changes. On a side note, I also fixed a couple of previously broken tests. All tests now pass.

The only somewhat worrying change is that I modified the constructor for `RedisPipeline`. It used to take host/port parameters, but now it just takes a Redis instance. This shouldn't be an issue, though, since I think `RedisPipeline` is never constructed directly and is only ever constructed via `from_spider` or `from_crawler`.

If you're okay with these changes, it would be great if you could pull them and publish an updated package.

Thanks & regards,
Chris