PHP 8.0 Support

Pull #3 updates composer constraints to include PHP 8.0 and add PHP 8.0 to the GitHub Actions build matrix

Postmark Client v4.0

Change composer constraints to allow v4 of the postmark client to be installed

Housekeeping…

• Update Doctrine Coding Standard and run phpcbf
• Allow Postmark Client ^2.9||^3.0
• Refactor tests so they no longer rely on Prophecy, using PHPUnit's MockObjects instead

Fix Component Installer

Fix composer.json to use 'config-provider' rather than 'component'

Initial Release

1.0.0

Add badges to Readme