Implement Serializable

pom.xml

@@ -4,7 +4,7 @@
   <groupId>com.yetanalytics</groupId>
   <artifactId>xapi-tools</artifactId>
   <packaging>jar</packaging>
-  <version>0.0.6</version>
+  <version>0.0.7</version>
   <name>xAPI Tools</name>
   <description>Java Serialization Model and Tools for xAPI Standard (IEEE 9274.1.1)</description>
   <url>https://github.com/yetanalytics/java-xapi-tools</url>

src/main/java/com/yetanalytics/xapi/model/JSONObject.java

@@ -1,5 +1,7 @@
 package com.yetanalytics.xapi.model;
 
-public interface JSONObject {
+import java.io.Serializable;
+
+public interface JSONObject extends Serializable {
     public boolean isEmpty();
 }

src/main/java/com/yetanalytics/xapi/model/LangTag.java

@@ -1,5 +1,6 @@
 package com.yetanalytics.xapi.model;
 
+import java.io.Serializable;
 import java.util.IllformedLocaleException;
 import java.util.Locale;
 import java.util.Locale.Builder;
@@ -14,7 +15,7 @@
  * allowing for retrieval of both the original String and the corresponding Locale.
  */
 @JsonInclude(Include.NON_NULL)
-public class LangTag {
+public class LangTag implements Serializable {
     private final String languageTagString;
     private final Locale languageTagLocale;
 

src/main/java/com/yetanalytics/xapi/model/XapiDuration.java

@@ -1,5 +1,6 @@
 package com.yetanalytics.xapi.model;
 
+import java.io.Serializable;
 import java.time.Duration;
 
 import com.fasterxml.jackson.annotation.JsonCreator;
@@ -12,7 +13,7 @@
 * allowing for retrieval of the original String.
 */
 @JsonInclude(Include.NON_NULL)
-public class XapiDuration {
+public class XapiDuration implements Serializable {
 
     /**
     * This constructor takes an 8601 formatted String and converts it to an