v0.9.0

• BC break: Split off individual components in order to stabilize core graph lib.
  (#120)
• Split off Algorithm namespace into separate graphp/algorithms package.
  (#119)
• Split off Exporter\TrivialGraphFormat into separate graphp/trivial-graph-format package.
  (#121)
• Split off Loader namespace into separate graphp/plaintext package.
  (#117)
• BC break: Remove Exporter from Graph and Graph::__toString() (trivial graph format exporter has been split off).
  (#122)
• BC break: Vertices can no longer be sorted by (in/out)degree (degree algorithm has been split off).
  (#128)
• Apply PSR-4 layout under src/ and add tests to achieve 100% test coverage.
  (#127 & #129)