Fix gecos and refactor email to be actually useful, update example

 * Commit nickcharlton/omniauth-pam@131b208 actually broke gecos as it always returned empty hash. Fixed...
 * email:
  * Change `:email` option to `:email_domain`. (This will brake backward compatibility, but I think this naming is better)
  * Do not add `:email` in `info` if no `:email_domain` is given (email in gecos will be preferred any case)
    * Previous code worked bit funny if no domain was given. Well only username works for local deliveries, but I do not think that is the case in applications
  * not sure if this should be done some where else (devise or similar omniauth interface), but I think for pam this seems ok
 * I think installing as less as possible system wide packages in example is better, thus only budler is needed. Updated example/README.md to reflect this. Also package managers should be used if possible.