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`
- `HasHeadersInterface`
- `HeaderValuesInterface`
- `RequestInterface`
- `ResponseInterface`
- `MessageFactoryInterface`
- `StreamInterface`.

The motivation for each interface and the design decisions that were made can be found in the "Design Decisions" section of the document.

Note: This pull request is a (mostly rewritten) fork of https://github.com/php-fig/fig-standards/pull/72.