Ensure backwards compatibility before modifying protocol

This change makes sure that things work smoothly when "we" are a newer
version than our peer and have more fields in our messages than they do.
Missing fields will be left at zero/nil.

(The other side will ignore our extra fields, for the same effect.)

Godeps/Godeps.json

@@ -23,7 +23,7 @@
 		},
 		{
 			"ImportPath": "github.com/calmh/xdr",
-			"Rev": "45c46b7db7ff83b8b9ee09bbd95f36ab50043ece"
+			"Rev": "214788d8fedfc310c18eca9ed12be408a5054cd5"
 		},
 		{
 			"ImportPath": "github.com/juju/ratelimit",

Godeps/_workspace/src/github.com/calmh/xdr/reader.go

@@ -154,6 +154,10 @@ func (e XDRError) Error() string {
 	return "xdr " + e.op + ": " + e.err.Error()
 }
 
+func (e XDRError) IsEOF() bool {
+	return e.err == io.EOF
+}
+
 func (r *Reader) Error() error {
 	if r.err == nil {
 		return nil