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

**NOTE**: Please only merge after first tagging as requested in #2. After merging, please tag v0.2.0.