Started fleshing out the UI topo model classes.
If reviewing this, please refer to http://tinyurl.com/onos-ui-topo-model

Change-Id: I4738392bec1a89c37dff15eff6fe04d66fcabd95
diff --git a/core/api/src/main/java/org/onosproject/ui/model/topo/UiDevice.java b/core/api/src/main/java/org/onosproject/ui/model/topo/UiDevice.java
index f267ca5..78f9d47 100644
--- a/core/api/src/main/java/org/onosproject/ui/model/topo/UiDevice.java
+++ b/core/api/src/main/java/org/onosproject/ui/model/topo/UiDevice.java
@@ -16,8 +16,17 @@
 
 package org.onosproject.ui.model.topo;
 
+import org.onosproject.net.Device;
+
 /**
  * Represents a device.
  */
 public class UiDevice extends UiNode {
+
+    private Device device;
+
+    @Override
+    protected void destroy() {
+        device = null;
+    }
 }