Bug fix to allow FlowId of 0.

* Added methods FlowId.isValid() and FlowEntryId.isValid()
* Change the default values of FlowId and FlowEntryId to -1 (invalid)
* Use the above isValid() methods as appropriate instead of explicitly
  comparing the IDs to hard-coded values.
diff --git a/src/main/java/net/onrc/onos/ofcontroller/util/FlowEntry.java b/src/main/java/net/onrc/onos/ofcontroller/util/FlowEntry.java
index 768a378..98dbd88 100644
--- a/src/main/java/net/onrc/onos/ofcontroller/util/FlowEntry.java
+++ b/src/main/java/net/onrc/onos/ofcontroller/util/FlowEntry.java
@@ -140,7 +140,7 @@
     public boolean isValidFlowId() {
 	if (this.flowId == null)
 	    return false;
-	return (this.flowId.value() != 0);
+	return (this.flowId.isValid());
     }
 
     /**
@@ -170,7 +170,7 @@
     public boolean isValidFlowEntryId() {
 	if (this.flowEntryId == null)
 	    return false;
-	return (this.flowEntryId.value() != 0);
+	return (this.flowEntryId.isValid());
     }
 
     /**