Add allow(...).to receive_message_chain
Remove unnecessary forwardable
Remove the unused receive_chained_messages
Change matcher_allowed to matcher_allowed? in targets.rb

This is more idiomatic. Also extract an allowed_matchers method.
Use allow_message instead of stub in stub_chain
Make receive_message_chain specs integration style
Remove the use of forwardable from receive_message_chain
Put the receive_message_chain features in their own file
Replace "mock" language with "double" language in the error generator.

Also updates specs that rely on these.