Paypal IPN handling

At first, yes, why not. But on a second thought, no. The point of entity builder is to define a set of rules for the *output*, let you know what to expect downstream. This has two parts, one is to validate that the input data contains everything we need (but not necessarily exactly in the format that comes out) and the other part is to coerce the input data where necessary to produce the expected output. But what we don't want to do is to say exactly from which kind of data the resulting output is created from. This addition would tie the builder to a foreign format.