Generalize response parsing code

This removes the many decodeXXXMessageSet() functions and replaces them
with a large map and a single generic decodeMessageSet() function. Also
change Responses to satisfy the Message interface as pointer types
(instead of the raw types), add the full set of top-level message sets
(though most of them still lack any message-parsing ability), adjust the
message set names to more closely mirror their OFX names, and fixup
tests and the command-line client to match the above changes.

cmd/ofx/ccdownload.go

@@ -49,7 +49,7 @@ func ccDownload() {
 		},
 		Include: true,
 	}
-	query.CreditCards = append(query.CreditCards, &statementRequest)
+	query.CreditCard = append(query.CreditCard, &statementRequest)
 
 	response, err := client.RequestNoParse(query)