Added `iter_spider_classes()` method to utils/spider.
Created SpiderManager class for easy spider lookup and creation.
Changed Engine initialization to better suit SpiderManager.
The problem was, that custom_settings was initialized too late in the
process.
Added global option to specify spider for the commands.
Default `get_spider()` in BaseCommand tries to intelligently pick the best match for spider.
Added `any_to_uri()` method.