Fix for Customer not showing in ajax search

This is a fringe issue that affects some user imports. If the user import does not populate the usermeta table, the Ajax customer search in WooCommerce will not return complete results.

The issue is due to the inner join on [line 1516 in class-wc-ajax.php](https://github.com/woothemes/woocommerce/blob/master/includes/class-wc-ajax.php#L1516), if the first_name or last_name meta_key doesn't exist for some reason (ie: users have been imported) then the query will not return any matches.

I have suggested a fix attached to this issue. It is further complicated by the current [poor performance of WP_Meta_Query](https://core.trac.wordpress.org/ticket/24093) so at the moment it is more efficient to do three separate queries rather than using 'relation' => 'OR' in the meta query ... I imagine this is why the pre_user_query function was added in WooCommerce in the first place.