Updated composer.json for Packagist
Added method and test to register a new account

You can now register for a new Mijn Pathé account using this library
Extended registration method to auto-update remaining data

This also meant moving the validation logic out of the Client and into the PersonalData object, which allowed me to re-use it for an update request, so now that's safer. I also changed the default values for gender and country to be MALE and NETHERLANDS, instead of NULL.
Updated README to include notice about registering for a new account