Initial release

Basic functionality required for receiving HTTP posts from Postmark and triggering appropriate events.

Comes with an example logging listener that you can inspect in order to create your own app specific listeners.

Future plans involve…

• Possibly supporting Zend Expressive if/when zend-expressive-authentication is released as it'll be required for basic HTTP auth.
• Maybe bundling factories for the official API client so it's one less job to do.
• Perhaps also bundling a ZF style mail transport adapter that communicates with the API instead of SMTP.