• Refactors MessageInterface to only provide getters.
• MessageInterface now defines getBody() to require that it return a
  StreamableInterface instance.
• Removes Request and Response interfaces
• Provides server-side interfaces:
  • IncomingRequestInterface, which provides accessors for HTTP
    properties and environment-specific items ($_SERVER, $_GET, $_POST,
    $_FILES, $_COOKIE, etc), and support for mutable "attributes".
  • OutgoingResponseInterface, which provides both accessors and
    mutators for all HTTP properties.
• Provides client-side interfaces:
  • OutgoingRequestInterface, which provides accessors and mutators for
    all HTTP properties.
  • IncomingResponseInterface, which provides accessors for HTTP
    properties.
• StreamableInterface removes attach().