[Workers] Add example worker

Add an example worker which inefficiently calculates
fibonacci numbers outside of the UI thread, for #12.

platform/execution/bundle.json

@@ -3,6 +3,7 @@
         "services": [
             {
                 "key": "workerService",
+                "implementation": "WorkerService.js",
                 "depends": [ "$window", "workers[]" ]
             }
         ]