Shopping Cart product quantity Add/Minus +- button?

I really like how the Marketpress shopping cart is working at the moment. I'm doing a site for someone and one of their requests is that the shopping cart has a small + button and a small - button next to each product in the shopping cart, where you can basically increase or decrease the quantity of each product in the shopping cart. Similar to the shopping cart of this site;jsessionid=0FA9D5F094BA953969D18D97FF8C8A28.dallas1?currentMenu=SHOP_SUPERMARKET

(You might have to click on a category and add a few products to see it in action)

Can someone please help me out with this request, I'd be really grateful if this could be solved.