[Workers] Add example worker

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

example/worker/src/FibonacciWorker.js

@@ -0,0 +1,15 @@
+/*global onmessage,postMessage*/
+(function () {
+    "use strict";
+
+    // Calculate fibonacci numbers inefficiently.
+    // We can do this because we're on a background thread, and
+    // won't halt the UI.
+    function fib(n) {
+        return n < 2 ? n : (fib(n - 1) + fib(n - 2));
+    }
+
+    onmessage = function (event) {
+        postMessage(fib(event.data));
+    };
+}());