Functionality to create 'Test Jobs' instead of real requests

Added functionality to pass 'test' as an option so that Postmark API handles the request as 'Test Job' and doesn't actually send the email(s). Useful for testing functionality in development environments. 

See: [Handling email in your test environment](http://blog.postmarkapp.com/post/913165552/handling-email-in-your-test-environment)