Fix README / PyPi description

Add markdown support for package description
Fix: PyPi publish

Bump django version
Add verbose: true to pypi script

Fix: update every occurences of 0.3.1 versions that prevented PyPi release

Removed support for Django 2
Added GitHub actions
Added Channels v3 compatibility
Added new module to support Channels > 1 #18

Fix: tests are running again
Fix: Correctly unsubscribe after on_start operation is complete

Breaking changes

If you are using this package with Django, read carefully the updated README as there might be breaking changes since the 0.3.x release.