Adding more unit tests.
Made some classes abstract which should have been.
diff --git a/core/api/src/main/java/org/onlab/onos/net/DefaultDevice.java b/core/api/src/main/java/org/onlab/onos/net/DefaultDevice.java
index 2c6c4ff..8b3eee1 100644
--- a/core/api/src/main/java/org/onlab/onos/net/DefaultDevice.java
+++ b/core/api/src/main/java/org/onlab/onos/net/DefaultDevice.java
@@ -51,7 +51,7 @@
 
     @Override
     public DeviceId id() {
-        return (DeviceId) super.id();
+        return (DeviceId) id;
     }
 
     @Override