Fix for window being undefined on Node.

This is related to the same issue MurhafSousli#6 where if you just try use window which is undeclared on Node then Angular Universal crashes.
A simple check makes sure window is not undefined, before using it.

src/service/window.service.ts

@@ -11,5 +11,5 @@ export class WindowService {
 
 function _window() : any {
   // return the global native browser window object
-  return window || undefined;
+  return typeof window != 'undefined' ? window : undefined;
 }