Fix grouped products sold individually add-to-cart

The add-to-cart button for products sold individually on a grouped
product page would always lead to the "Please choose a product to
add to your cart" error. This fixes that, and also restores th
'single_add_to_cart_text' filter to the button's text.
Restore single_add_to_cart_button class
Merge pull request #2593 from thenbrent/group-product-fix

Fix  add-to-cart for grouped products which are sold individually