Add `Orchestra\Notifier\Message::create()` helper.

Signed-off-by: crynobone <crynobone@gmail.com>
Simplify `Orchestra\Notifier\NotifiableTrait`.

Signed-off-by: crynobone <crynobone@gmail.com>
Fixes indentation.

Signed-off-by: crynobone <crynobone@gmail.com>
Fixes testcase and improve docblock.

Signed-off-by: crynobone <crynobone@gmail.com>
Merge branch '2.2'