Big sync from internal version:
          - include MessageSet support code
          - support message_set_wire_format for extensions
          - use append throughout encode.go

R=r
CC=golang-dev
http://codereview.appspot.com/3023041
diff --git a/proto/text_parser.go b/proto/text_parser.go
index 61513f8..009b3a7 100644
--- a/proto/text_parser.go
+++ b/proto/text_parser.go
@@ -33,7 +33,7 @@
 
 // Functions for parsing the Text protocol buffer format.
 // TODO:
-//     - groups.
+//     - message sets, groups.
 
 import (
 	"fmt"