hooks for security and customization

In addition to the added hooks, I am proposing to change the name and position of register_form to woocommerce_register_form, since WordPress already uses register_form. The woocommerce_register_form allows that specific form to be targeted.

The other hooks for the forms allow custom messages, scripts, and security functions to handle processing, and stop the process if needed.

The woocommerce_before_delete_order_items hook allows other plugins/extensions to cleanup order specific items if an order is deleted. I use this in my API Manager extension.