Fix woocommerce_add_order_item() return value & improve 'woocommerce_new_order_item' params.

If a plugin hooks to `'woocommerce_new_order_item'` and run some sort of `$wpdb` insert query or calls a function which runs an insert query, like `woocommerce_add_order_item_meta()`, then the item ID returned by `woocommerce_add_order_item()` will actually be the ID of a different DB entry (because `woocommerce_add_order_item()` returns `$wpdb->insert_id`). 

This pull request fixes that.

It also adds extra params for the 'woocommerce_new_order_item' hook.