Sync interfaces with recent changes in proposal

- [php-fig/fig-standards#330] Add attach to stream interface
- [php-fig/fig-standards#331] Clarify return value of stream interface detach()
  method
- [php-fig/fig-standards#332] Add getMetadata() to stream interface
- [php-fig/fig-standards#333] Remove $maxLength parameter from stream interface
  getContents() method
- [php-fig/fig-standards#334] Rename StreamInterface to StreamableInterface
Merge pull request #3 from weierophinney/hotfix/sync-to-proposal

Sync interfaces with recent changes in proposal