Method initialize

* Initializes the database link $_db with its variables * * Creates a new mysqli object and stores that database link for use in the * other static methods of the class. * * @param string $db_URL the string of the database's URL to connect to * @param string $db_username the string of the database username * @param string $db_password the string of the database password

(
    $db_URL, $db_username, $db_password, $db_name, $db_salt
  )