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.

signup_test.go

@@ -150,7 +150,7 @@ func TestUnmarshalAcctInfoResponse(t *testing.T) {
 			BankAcctInfo: &bankacctinfo,
 		}},
 	}
-	expected.Signup = append(expected.Signup, acctInfoResponse)
+	expected.Signup = append(expected.Signup, &acctInfoResponse)
 
 	response, err := ofxgo.ParseResponse(responseReader)
 	if err != nil {