Allow for use of settings on payment methods.

[MatthewKennedy]

This PR allows developers to use Active Store settings instead of preferences for new payment methods.

Add the following method to your payment method gateway model

    # Set to true to load in the view file from views/spree/admin/shared/payment_methods/_your_gateway.html.erb
    def use_settings?
      true
    end

Existing payment methods without this method will continue to use preferences.