==========
  * new: better error handling: `show()` now returns a result instead of panicking
  * added: server deserializes notifications
  * fixed: hint types other than just strings
  * added: convenience from::<str>() for Urgency
  * added: Urgency::from(&str) for convenience
  * renamed urgencies: Low, Normal and Critical (as in both standards)
  * Tidying up
  * more documentation
  * added: internal `stop_server()`
  * fixed: urgency in example
  * tiny things, better looking Rust