Ability to enable/disable payment on a per node basis

This patch adds the ability to enable/disable payments on a per node basis. Rather than have all Signup nodes require payment this allows you to choose when you are creating a new node with signup enabled if you would like to require payment or not.

I liked this method better than just setting the price to zero. It also means that people are not shown "Price is $0.00" please choose a payment method", which is just confusing.

Also adds an option the settings page that allows you to choose wether to enable/disable payment by default on new nodes.