Pay Node should only list embeddable forms for use on nodes

I'm not 100% sure on what the "embeddable" property's intended use is for classes of type pay_form, but I'm assuming it's intended to indicate if the form may be used in forms or pages regardless of location. The Webform Pay module I'm writing also defines a subclass of pay_form, but it won't work anywhere except for within webforms (since information like the payment "total" come from other parts of the form). So I've set embeddable to FALSE for this class, but it still shows up at admin/settings/pay/node as an option for embedding within nodes.

This patch creates an instance of each possible form class and checks that it is embeddable before presenting it as an option. I'm not sure if this is the correct use of embeddable or not.