Improved Add to cart AJAX functionality to support quantity parameter.

Added backend support for a `quantity` parameter to ''Add to cart'' AJAX functionality in `woocommerce-ajax.php` and `add-to-cart.js`, like it is already supported in `woocommerce-functions.php` under `woocommerce_add_to_cart_action()`.

It is expected that the normal action by visiting a link behaves the same as by sending it through AJAX. This backend modifications allow developers to have an intuitive way to control the quantity of products added to the cart with a link or  AJAX request (eg. use the filter `woocommerce_loop_add_to_cart_link` to modify Add to cart buttons or create their own shortcut buttons for higher quantities).