[PSR-7] Add false as a return value for read()

Per @mtdowling, `StreamableInterface::read()` should allow returning `false`.
Merge pull request #342 from weierophinney/hotfix/psr7-read-return

[PSR-7] Allow returning false from StreamableInterface::read()
[PSR-7] Clarify MessageInterface::getHeaders()

- Per [the mailing list](https://groups.google.com/d/msg/php-fig/H_NR7qctw-8/9jDffi4_j7cJ), clarified the return value of `getHeaders()` to indicate the structure of the returned array.