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/invdownload.go

@@ -58,7 +58,7 @@ func invDownload() {
 		Include401K:    true,
 		Include401KBal: true,
 	}
-	query.Investments = append(query.Investments, &statementRequest)
+	query.InvStmt = append(query.InvStmt, &statementRequest)
 
 	response, err := client.RequestNoParse(query)