Made TrackingId serializable

dddsample/src/main/java/se/citerus/dddsample/domain/model/cargo/TrackingId.java

 import org.apache.commons.lang.Validate;
 import se.citerus.dddsample.domain.shared.ValueObject;
 
+import java.io.Serializable;
+
 /**
  * Uniquely identifies a particular cargo. Automatically generated by the application.
  *  
  */
-public final class TrackingId implements ValueObject<TrackingId> {
+public final class TrackingId implements ValueObject<TrackingId>, Serializable {
 
   private String id;