fix 'check-available-stock' #533 and fix #526

fix issue #533 and #526

test 'check-available-stock'  on template 'product.html', 'single-product.html' and 'cart.html'
test $QUANTITY on template 'product.html'
add config variable 'quantity-stock-default'

fix error if stock = 0 and  'check-available-stock' = 0 in Action/Order.php