fix support for nodejs

src/core/Engine.js

@@ -49,10 +49,6 @@ var Engine = {};
                 isFixed: false,
                 frameRequestId: 0
             },
-            render: {
-                element: element,
-                controller: Render
-            },
             broadphase: {
                 controller: Grid
             }
@@ -60,7 +56,17 @@ var Engine = {};
         
         var engine = Common.extend(defaults, options);
 
-        engine.render = engine.render.controller.create(engine.render);
+        if (element && !engine.render) {
+            engine.render = {
+                element: element,
+                controller: Render
+            };
+        }
+
+        if (engine.render && engine.render.controller) {
+            engine.render = engine.render.controller.create(engine.render);
+        }
+
         engine.world = World.create(engine.world);
         engine.pairs = Pairs.create();
         engine.broadphase = engine.broadphase.controller.create(engine.broadphase);

src/core/Runner.js

@@ -13,6 +13,11 @@ var Runner = {};
 
 (function() {
 
+    if (typeof window === 'undefined') {
+        // TODO: support Runner on non-browser environments.
+        return;
+    }
+
     var _fps = 60,
         _deltaSampleSize = _fps,
         _delta = 1000 / _fps;