trigger_custom_event and merge_table_records_with_pk api's integration

Not sure when you exactly forked the project but we modified the way we handle the messages. We use a helper method now : `Responsys::Helper.get_message("api.object.custom_event.one_of_event_name_or_event_id_is_required")`.

You can either rebase the newest changes from master on your branch or I can do it. I'll add more comments about them to help in the next ones.