Improve address structure (eg de-duplicate zipcode and postal code)

[philipashlock]

There is some redundancy in the spec for including a zipcode/postalcode. There is a standalone field for zipcode but the documentation for address also notes that it can include the postal code. Perhaps the address should be required to be more structured with the primary components separated into individual fields. We could use http://schema.org/PostalAddress as a reference.