Contribution Pages should support optional user selection of available payment processors

Many organizations support more than one online payment method. For example, we support PayPal and TrustCommerce (a credit card processor).

Right now, because each contribution form can only have one payment processor, we have to make two forms, and the user has to click a click to get to the other form if the default isn't the one they want.

Users are accustomed to radio buttons or drop-downs as the way to choose payment method, in most donation or e-commerce experiences.

Probably the way to do this is, when creating a Contribution Page, we should be able to mark which payment processors should be available for that form.