Add REST API for a device.
In this commit, only added /wm/onos/topology/devices/json to get all device information.

Change-Id: I9a56be1335914ba965f1e921960d3cabf813c61c
diff --git a/src/main/java/net/onrc/onos/core/topology/NetworkGraph.java b/src/main/java/net/onrc/onos/core/topology/NetworkGraph.java
index 99f7eda..d450784 100644
--- a/src/main/java/net/onrc/onos/core/topology/NetworkGraph.java
+++ b/src/main/java/net/onrc/onos/core/topology/NetworkGraph.java
@@ -83,4 +83,6 @@
      * Release the read lock on the topology.
      */
     public void releaseReadLock();
+
+    public Iterable<Device> getDevices();
 }