fix for meta data creation and allow sso user to log out

This is a pull request with fixes for a coupe of problems I had:

1) The plugin was not creating meta data for sso users, such as `first_name` and `last_name`.  This may be because I was deleting the user between attempts and wordpress doesn't clean up the meta data.  The result was that the meta data was being set to an array (e.g. ['Bob']) rather than a single field (e.g. 'Bob').  Whatever the reason, using `wp_insert_user` to create the user with the meta data prevented this from being a problem.

2) As far as I can tell, log out didn't work with this plugin, so I added some code to clear the saved cookies on log out.  This seems to trigger the log out.

Thanks for considering these changes and for sharing your plugin.