Added basic ga events for add to cart, customer login, customer registration

[lucho2d7]

Added the ga event send calls for these Magento events:

• controller_action_predispatch_checkout_cart_add
  Sends event with category=catalog, action=add-to-cart, label=[productId]
• customer_login
  Sends event with category=customer, action=login, label=[customerId]
• customer_register_success
  Sends event with category=customer, action=registration, label=[customerId]

[fooman]

Thanks @lucho2d7 for your contribution. The changes unfortunately no longer cleanly apply to the latest master branch. Would you mind checking the merge conflicts and rebase your PR to resolve. Thanks.