Update with specific details regarding the new option and the use of required sessions.
Remove unneeded check. This was fixed in WP 3.3.0 and now just leads to error messages in the Apache log due to the value of $wpdb->sitemeta
Errors are similar to:

[Thu Apr 03 21:03:06 2014] [error] [client XX.XX.XX.XX] WordPress database error You have an error in your SQL syntax; check the
manual that corresponds to your MySQL server version for the right syntax to use near
'WHERE meta_key = 'shibboleth_logout_url' AND site_id = 0' at line 1 for query SELECT meta_value FROM  WHERE meta_key = 'shibboleth_logout_url' AND site_id = 0 made by activate_plugin, do_action('activate_shibboleth/shibboleth.php'),
call_user_func_array, shibboleth_activate_plugin, shibboleth_add_option,
referer: http://wpdev.hvcc.edu/wordpress/wp-admin/plugins.php?deactivate=true&plugin_status=all&paged=1&s=

Note the blank space between FROM and WHERE.
Merge pull request #2 from billjojo/master

readme and add_site_option fix, props billjojo