0.4.0

• Removed references to RFC 2616 (php-fig/fig-standards#346)
• Added @throws annotations (php-fig/fig-standards#345)
• Clarify MessageInterface::getHeaders() (php-fig/fig-standards#347)
• Whitespace!!! (php-fig/fig-standards#351)
• Clarify MessageInterface header methods (php-fig/fig-standards#353)
• Clarify superglobal usage (php-fig/fig-standards#354)
• Clarifications in language. (php-fig/fig-standards#356)