Pullkin

Change:

* Remove access to classmethod `register()`. Available only via an instance of `(Aio)Pullkin`

Fix

* Fix closed client

Fix

* Classmethod problems

Changed and fix

* `TypeError: object of type 'NoneType' has no len()` -> `ConnectionError: Error during request: {e}` with logging

Fix

* `'NoneType' has no len()`

Changed

* Move Notification model to new `models` directory
* Rename Notification -> Message
* Add example to `README.md`

Remove

* Unused `publish.py` script

Add

* Call `AioPullkin.callback` if no one handler was registered

Changed

* Update versions of dependencies
* Move build and dev dependencies to `requirements-dev.html`

Fix

* Sync handlers are now called directly with arguments, rather than with a tuple of arguments

Remove

* Compatible with Francesco (`pullkin.old`) version