[PSR-7] Add attach() to the StreamInterface

- Per Evert Pot on the mailing list, having an attach() method can be useful for
  replacing the underlying stream resource -- particularly when in server-side
  code and you wish to change from one content stream to another (e.g., to
  return a file instead of a string).
Merge pull request #330 from weierophinney/feature/stream-attach

[PSR-7] Add attach() to the StreamInterface