Avoiding redefinition of core function wp_notify_moderator

This patch is to replace the redefinition of a core function by adding a filter for comment notification recipients.
Instead of redefining wp_modify_moderator() only to be able to get the co-authors e-mails, I've added a filter to the plugin, "filter_comment_moderation_email_recipients", which filters the comment moderation recipient list and adds the co-authors e-mails.

On WP 3.7.0, a new filter was applied to the core wp_modify_moderator() function ("comment_moderation_recipients"), and the lack of the application of this filter on the modified plugin version was preventing other plugins that use "comment_moderation_recipients" from working.

Probably the same approach can be used on the wp_notify_postauthor() function on this plugin on a future work.