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/bankdownload.go

@@ -55,7 +55,7 @@ func download() {
 		},
 		Include: true,
 	}
-	query.Banking = append(query.Banking, &statementRequest)
+	query.Bank = append(query.Bank, &statementRequest)
 
 	response, err := client.RequestNoParse(query)
 	if err != nil {