Add support for WooCommerce Subscriptions

Hi Vlado,

I’ve been having trouble testing Subscriptions on my local server. I haven’t tested this extensively, but I think you can add this to the dummy gateway’s constructor and the gateway will support WooCommerce Subscriptions, Deposits, and refunds. At the very least, I am able to complete a test order with a subscription in the cart, which is what I needed for the moment now that the stripe plugin requires PHP5.6.

        $this->supports = array(
            'products',
            'refunds',
            'deposits',
            'subscriptions',
            'subscription_cancellation',
            'subscription_reactivation',
            'subscription_suspension',
            'subscription_amount_changes',
            'subscription_payment_method_change', // Subs 1.n compatibility.
            'subscription_payment_method_change_customer',
            'subscription_payment_method_change_admin',
            'subscription_date_changes',
            'multiple_subscriptions',
        );

cheers,
-kathy