Paypal IPN handling

Hmm... again, some ideas to extend the builder comes to my mind... Should we have a third validation, in addition to `:mandatory` and `:optional`, that would validate that either `:order_id` or `:authorization_id` is present?

```
EntityUtils.define_builder(
  [:first, :string, :optional],
  [:second, :string, mandatory_if_nil: :first]
```

...hmh, maybe not.