added use_helper to README
also added a warning about use of try_first_pass with use_helper
default options for pam_google_authenticator and google-authenticator can now be specified in the /etc/google-authenticator.conf configuration file
The google-authenticator utility now accepts ${USER} and ${HOME} in the 'secret' option
The google-authenticator utility will chown the secret file according to the helper_owner option in the config file if the use_helper option is also enabled in the config file.