Add an experimental protobuf serializer

Provide a core protobuf serializer that can either write objects with
an envelope (a 4 byte prefix and a runtime.Object) or raw to a byte
array.

pkg/runtime/types.go

@@ -38,8 +38,6 @@ type TypeMeta struct {
 
 const (
 	ContentTypeJSON string = "application/json"
-	// TODO: Fix the value.
-	ContentTypeProtobuf string = "application/protobuf"
 )
 
 // RawExtension is used to hold extensions in external versions.