Adding support for Office Postal Codes

In addition to monthly updates this release adds support for 大口事業所の個別番号, or postal codes for high-volume office destinations.

While these codes look the same as other codes, the data associated with them is very different, and includes an explicity address and recipient name. You can access them with the same posuto.get you're familiar with, but the returned object may be an OfficeCode instead of a PostalCode, so be sure to check.

See #4 for further details on this feature.