Add an option to index as another user instead of only anonymous

Adds an option, which enables you to chose to index using the superadmin user instead of the anon user. The default case ( anon indexing ) is not changed of course. I think the motivation is clear, because for sites with a lot of permissions it counterfires the "access implementation" of solr. Instead of indexing everything and then use modify_query to alter the solr-search query that way, that it respects the user permissions, the anon indexing just takes care of "well only show what _everybody" can see and only index that - no matter what other users could search for.

I did not place the option on the administration page yet, just let me know were you think it fits best and i add it.