Refactor DSL and Artifacts.

Library/Homebrew/cask/lib/hbc/artifact/abstract_artifact.rb

...y/Homebrew/cask/lib/hbc/artifact/abstract_flight_block.rb

Library/Homebrew/cask/lib/hbc/audit.rb

Library/Homebrew/cask/lib/hbc/container/base.rb

Library/Homebrew/cask/lib/hbc/dsl.rb

Library/Homebrew/cask/lib/hbc/dsl/appcast.rb

Library/Homebrew/cask/lib/hbc/dsl/base.rb

Library/Homebrew/cask/lib/hbc/staged.rb

Library/Homebrew/test/cask/dsl_spec.rb