Updates to get_tracking_info

Get rid of #message in shipment event constructor and replace it with #status - #status should return a symbol that clients can switch on reliably. #message didn't appear to be used anywhere, and since ShipmentEvent feels like a private API we figured this was a good way to go, even though it breaks backwards compatibility very slightly.

Add get_tracking_info to the USPS carrier - it supports almost everything the UPS version does, missing only destination location and extended address information in shipment events.