Constructor for the ftp_connection class.

• Example: $package_ftp = new ftp_connection('server', 21, 'username', 'password');
• Will not work if fsockopen or fwrite are disabled.
• Checks for valid response codes after each step.
• Sets a 'bad_server' error and returns if the connection couldn't be opened
• Sets a 'bad_response' error and returns if it does not receive a valid "welcome message" from the server
• Sets a 'bad_username' error and returns if the username isn't valid
• Sets a 'bad_password' error and returns if the password isn't valid