Make sure factorial is defined in intro

Author: marijnh

code/intro.js

@@ -18,3 +18,11 @@ function sum(array) {
     total += array[i];
   return total;
 }
+
+function factorial(n) {
+  if (n == 0) {
+    return 1;
+  } else {
+    return factorial(n - 1) * n;
+  }
+}