lib/media_wiki/gateway.rb (send_request): New method.

Changes (undocumented) return value of some methods.
spec/{ -> lib/media_wiki/}fake_media_wiki.rb: Moved to main project space.
bin/mediawiki-gateway: New command-line client merging previous sample scripts.

Dropped `MediaWiki::Config` and `hosts.yml`, changed some options/argument handling.