code_id and user_id in a separate db table

I have changed the plugin to store the relation of user_id and code_id in a separate table instead of using one entry in the options table. This should scale much better if you expect several hundred users, who are using the sponsored membership.

I did not succeed using the dbDelta() function to create the table, so I used a simple $wdpb->query() instead. Also, there is no conversion to import the old option setting into the table (yet). So there should be a warning for users who want to update from an older version (or there should be an import function).