making SessionData serializable so we can turn it into (encrypted)

session ids when need.

u2f-ref-code/java/src/com/google/u2f/server/data/SecurityKeyData.java

@@ -13,7 +13,6 @@ public class SecurityKeyData {
   private final X509Certificate attestationCert;
 
   public SecurityKeyData(byte[] keyHandle, byte[] publicKey, X509Certificate attestationCert) {
-    super();
     this.keyHandle = keyHandle;
     this.publicKey = publicKey;
     this.attestationCert = attestationCert;

u2f-ref-code/java/src/com/google/u2f/server/data/SessionData.java

 package com.google.u2f.server.data;
 
-public class SessionData {
+import java.io.Serializable;
+
+public class SessionData implements Serializable {
+  private static final long serialVersionUID = 1750990095756334568L;
+
   private final String accountName;
   private final byte[] challenge;
   private final String appId;