human readable time representation for toString

Example:
 LinkEvent{time=2014-10-28T23:24:23.461, ...

Change-Id: I439cea488afd97dfa4f1c1e5770f2e2d7b64d321
diff --git a/core/api/src/main/java/org/onlab/onos/mastership/MastershipEvent.java b/core/api/src/main/java/org/onlab/onos/mastership/MastershipEvent.java
index 74f8161..1aecfb4 100644
--- a/core/api/src/main/java/org/onlab/onos/mastership/MastershipEvent.java
+++ b/core/api/src/main/java/org/onlab/onos/mastership/MastershipEvent.java
@@ -15,6 +15,7 @@
  */
 package org.onlab.onos.mastership;
 
+import org.joda.time.LocalDateTime;
 import org.onlab.onos.cluster.RoleInfo;
 import org.onlab.onos.event.AbstractEvent;
 import org.onlab.onos.net.DeviceId;
@@ -83,7 +84,7 @@
     @Override
     public String toString() {
         return MoreObjects.toStringHelper(getClass())
-                .add("time", time())
+                .add("time", new LocalDateTime(time()))
                 .add("type", type())
                 .add("subject", subject())
                 .add("roleInfo", roleInfo)