Progress so far on the settings api changes of the GSoC project.
Settings freeze support and per-spider settings will be implemented on separate pull requests.
This early adaptation of the new Settings interface is needed for running tests
Explicit conversion of FEED_EXPORTERS and FEED_EXPORTERS_BASE values into dictionaries, and cast of the keys to a list for py3 support.
hi @curita, this PR looks perfect, I was about to merge it for 0.24 but realized it doesn't offer backwards compatibility for deprecated