Incorporate feedback about selective mutability

- Rewrites the message interface to be immutable
- Separates interfaces into client-side and server-side pairs
  - client side request is mutable, but response is immutable
  - server side request is immutable, but response is mutable
Remove StreamableInterface::attach()

- Removes the attach() method, per https://groups.google.com/d/msgid/php-fig/01e87630-2976-4824-956d-61e7506550df%40googlegroups.com
Updated URL properties

- URLs in messages MUST be strings.
- URLs SHOULD be absolute but MAY be relative.
- See:
  - https://groups.google.com/d/msgid/php-fig/d794ec6d-79be-4b85-9f7d-b678be6c59d3%40googlegroups.com
    (strings, not objects)
  - https://groups.google.com/d/msgid/php-fig/d794ec6d-79be-4b85-9f7d-b678be6c59d3%40googlegroups.com
    (allow relative)
Require that getBody() MUST return a StreamableInterface

- Per https://groups.google.com/d/msgid/php-fig/d794ec6d-79be-4b85-9f7d-b678be6c59d3%40googlegroups.com
[#349] Clarify header casing

- Particularly that the case insensitivity refers to the header _names_ only.
- Close [#349]
Revised meta document

- Better detail in summary of what HTTP messages are, and how we plan to model
  them.
- Detail client-side vs server-side usage of HTTP messages, and why we have
  split the interfaces into the different contexts.
Merge pull request #366 from weierophinney/feature/selective-mutability

[PSR-7] Selective Mutability