Settings priorities dictionary
New set and setdict method using SettingsAttribute in Settings
Deprecate CrawlerSettings, as its functionality is replicable by Settings class
Settings and SettingsAtribute tests
Fix and extend the documentation of the new Settings api
Settings topic updated
Fix settings repr on the logs of the shell and tutorial docs topics
setmodule helper method on Settings class
Update get_crawler method in utils/test.py with new Settings interface

This early adaptation of the new Settings interface is needed for running tests
get_project_settings now returns a Settings instance
Change command's default_settings population in cmdline.py
Change how settings are overriden in ScrapyCommands
Fix settings usage in runspider and crawl commands

Explicit conversion of FEED_EXPORTERS and FEED_EXPORTERS_BASE values into
dictionaries, and cast of the keys to a list for py3 support.
Fix settings usage across tests