Add additional class to add to cart button and change action in js

When an out of stock product is shown (http://cld.wthms.co/Mxdh) the button still has the add_to_cart_button class which is used by the js to add products to the cart, that results in this message http://cld.wthms.co/SSmx

This PR adds add_to_cart_action to buttons where the product is in stock and changes the class used to trigger the on-click to add_to_cart_action