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/NetworkGraphImpl.java b/src/main/java/net/onrc/onos/core/topology/NetworkGraphImpl.java
index 115afb5..b20ab6b 100644
--- a/src/main/java/net/onrc/onos/core/topology/NetworkGraphImpl.java
+++ b/src/main/java/net/onrc/onos/core/topology/NetworkGraphImpl.java
@@ -105,6 +105,11 @@
         return mac2Device.get(address);
     }
 
+    @Override
+    public Iterable<Device> getDevices() {
+        return Collections.unmodifiableCollection(mac2Device.values());
+    }
+
     protected void putDevice(Device device) {
         mac2Device.put(device.getMacAddress(), device);
     }