HTTP Message Proposal (take 2)

This proposed PSR provides interfaces that can be used for representing HTTP messages.

This PR provides the following interfaces:

- `MessageInterface`
  - `RequestInterface`
  - `ResponseInterface`
- `HeaderFieldValuesInterface`
- `StreamInterface`

The motivation for each interface and the design decisions that were made can be found in the "Design Decisions" section of the document. This PR probably needs some editing and clarifications in the prose, but I think the actual interfaces are pretty well fleshed out.

Note: This pull request is a (mostly rewritten) fork of https://github.com/php-fig/fig-standards/pull/72.