added tooltip hint to expiration date field and CVV code field

Occasionally I am still getting support tickets asking if we could add some sort of hint/description for the credit card fields especially the CVV/card code field as some customers still don't know what that is.  

So this commit adds jQuery tiptip to checkout and added the hint text to the expiration and CVV code fields.  Be sure to compile the woocommerce sass file to see it in affect.