Added unit tests for the event abstractions.
Added Element as the notion of common ancestry between Device and Host.
diff --git a/net/api/src/main/java/org/onlab/onos/net/Element.java b/net/api/src/main/java/org/onlab/onos/net/Element.java
new file mode 100644
index 0000000..c11d103
--- /dev/null
+++ b/net/api/src/main/java/org/onlab/onos/net/Element.java
@@ -0,0 +1,17 @@
+package org.onlab.onos.net;
+
+import org.onlab.onos.net.provider.Provided;
+
+/**
+ * Base abstraction of a network element, i.e. an infrastructure device or an end-station host.
+ */
+public interface Element extends Provided {
+
+ /**
+ * Returns the network element identifier.
+ *
+ * @return element identifier
+ */
+ ElementId id();
+
+}