Add Citrus integration

This is a cleaned up and working version of #796 which adds an integration for the Citrus gateway. Tested and working with Shopify.

I had to make a small change to the action_view_helper to allow for integrations to specify a `credential_based_url` method as alternative to the standard `service_url`. This was needed because the citrus url depends on the merchant's credentials.

@odorcicd @jduff 
/cc @louiskearns