Added missing javadocs. Created a separate class for message subjects used by distribued device store
diff --git a/core/store/dist/src/main/java/org/onlab/onos/store/device/impl/InternalPortEvent.java b/core/store/dist/src/main/java/org/onlab/onos/store/device/impl/InternalPortEvent.java
index 327c185..48e3be6 100644
--- a/core/store/dist/src/main/java/org/onlab/onos/store/device/impl/InternalPortEvent.java
+++ b/core/store/dist/src/main/java/org/onlab/onos/store/device/impl/InternalPortEvent.java
@@ -7,6 +7,10 @@
 import org.onlab.onos.net.provider.ProviderId;
 import org.onlab.onos.store.common.impl.Timestamped;
 
+/**
+ * Information published by GossipDeviceStore to notify peers of a port
+ * change event.
+ */
 public class InternalPortEvent {
 
     private final ProviderId providerId;