fixed broken author+term query

when co-author-plus plugin is enabled, we noticed that we were not getting results when searching for author and another taxonomy term. it looks like co-author-plus is actually not adding the join term it needs when a WP query also contains a taxonomy query, because the taxonomy query generates a join clause that's identical to what co-author-plus generates and co-author-plus cannot tell whether that clause is a duplicate or not.

the fix makes the co-author-plus-generated sql join clause slightly different (by using an alias for the wp_term_relationships table). i think this does not change the functionality of the plugin but avoids confusing sql clauses from different sources.