Add extensions to the generator.

R=r
CC=golang-dev
http://codereview.appspot.com/774041
diff --git a/proto/extensions.go b/proto/extensions.go
index 480e8d3..3726d3c 100644
--- a/proto/extensions.go
+++ b/proto/extensions.go
@@ -128,7 +128,7 @@
 	return unsafe.Unreflect(t, base), nil
 }
 
-// TODO(: (needed for repeated extensions)
+// TODO: (needed for repeated extensions)
 //   - ExtensionSize
 //   - AddExtension