Make Groups optional (via module_exists())

When using the walkhub module standalone and editing a walkthrough (I was importing from selenium), if you do not have Organic Groups enabled, PHP will be rather upset:

PHP Fatal error:  Call to undefined function og_get_entity_groups() in /var/www/example/sites/all/modules/walkhub/walkhub.pages.inc on line 139, referer: http://example.com/walkthrough/import

This simply wraps calls (and some form_state items) referencing og functions in a if (module_exists('og')) {things}.