Refactor the FlowManager addFlow() API:
 * It returns the FlowID on success, otherwise null
 * If the FlowID of the FlowPath is not set (i.e., its value is -1),
   the FlowManager will assign it.
 * Modified the web/add_flow.py usage to explain the value of Flow ID of -1
 * Removed method addAndMaintainShortestPathFlow() from the FlowManager,
   because it is not needed anymore.
diff --git a/src/main/java/net/onrc/onos/ofcontroller/flowmanager/web/AddFlowResource.java b/src/main/java/net/onrc/onos/ofcontroller/flowmanager/web/AddFlowResource.java
index 0926f91..9afaaec 100644
--- a/src/main/java/net/onrc/onos/ofcontroller/flowmanager/web/AddFlowResource.java
+++ b/src/main/java/net/onrc/onos/ofcontroller/flowmanager/web/AddFlowResource.java
@@ -64,9 +64,9 @@
 
 	// Process the request
 	if (flowPath != null) {
-	    if (flowService.addFlow(flowPath, result) != true) {
-		result = new FlowId();		// Error: Return empty Flow Id
-	    }
+	    FlowId addedFlowId = flowService.addFlow(flowPath);
+	    if (addedFlowId != null)
+		result = addedFlowId;
 	}
 
         return result;