Add validations subscriptions (RIPD-504)

test/jsonrpc-test.js

@@ -169,4 +169,20 @@ suite('JSON-RPC', function() {
       done();
     });
   });
+
+  test('subscribe validations', function(done) {
+    var rippled_config = testutils.get_server_config(config);
+    var client         = jsonrpc.client("http://" + rippled_config.rpc_ip + ":" + rippled_config.rpc_port);
+    var http_config    = config.http_servers["zed"];
+
+    client.call('subscribe', [{
+      'url' :  "http://" + http_config.ip + ":" + http_config.port,
+      'streams' : [ 'validations' ],
+    }], function (result) {
+      // console.log(JSON.stringify(result, undefined, 2));
+      assert(typeof result === 'object');
+      assert(result.status === 'success');
+      done();
+    });
+  });
 });