Registration.php - code improvement

Rather than directly use $_POST and $_GET inside functions of the class, I have added some parameters to class functions.

It's far more easy to know now input data needed by each functions 
- registerNewUser($user_name, $user_email, $user_password, $user_password_repeat, $captcha)
- verifyNewUser($user_id, $user_activation_hash)
- sendVerificationEmail($user_id, $user_email, $user_activation_hash)

Some private variables have been removed because not use anymore.

At user creation, I record inside database the current datetime and the ip address used