New set and setdict method using SettingsAttribute in Settings
Change basestring for six.string_types in Settings method for py3 support
Deprecate CrawlerSettings, as its functionality is replicable by 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
Settings and SettingsAtribute tests
Fix and extend the documentation of the new Settings api