[PSR-7] Rename StreamInterface to StreamableInterface

- @auroraeosrose raised the point that the name `StreamInterface` connotes that
  the object can be consumed as a PHP stream (i.e., that it acts as a stream
  wrapper). Since it does not, renaming it to `StreamableInterface` makes sense
  as it indicates the contents are something that can be streamed, but that the
  implementation may not be an actual PHP stream.
Merge pull request #334 from weierophinney/feature/streamable-interface-renaming

[PSR-7] Rename StreamInterface to StreamableInterface