Add user to list request value object

`$expertSender->addUserToList()` has got a lot arguments and the number will grow (it's already missing `$force`, `$name`, `$force`, `$trackingCode`, `$vendor` attributes).

I created `AddUserToListRequest` class which should ease creating subscriptions. Instead of passing gazillion arguments to `addUserToList()`, one could call `addUserToListRequest($request)`, where `$request` is a simple value object.

Example:

    $request = (new AddUserToListRequest())
        ->setEmail($email)
        ->setListId($listId)
        ->freeze();
    $expertSender->addUserToListRequest($request);

It also validates required attributes.

Few things could be changed, I've mentioned them in commit's code comments.