Default to no-op callbacks for #halt methods

lib/cylon.js

@@ -105,6 +105,7 @@ Cylon.config = function(key) {
 //
 // Returns nothing
 Cylon.halt = function halt(callback) {
+  callback = callback || function() {}
   // if robots can't shut down quickly enough, forcefully self-terminate
   var timeout = Config.halt_timeout || 3000
   Utils.after(timeout, callback);

lib/robot.js

@@ -285,6 +285,8 @@ Robot.prototype.startDevices = function(callback) {
 //
 // Returns nothing
 Robot.prototype.halt = function(callback) {
+  callback = callback || function() {};
+
   var fns = [];
 
   for (var d in this.devices) {