Add JavaScript to benchmark runner.

benchmark/fib.js

@@ -0,0 +1,13 @@
+function fib(n) {
+  if (n < 2) return n;
+  return fib(n - 1) + fib(n - 2);
+}
+
+var start = process.hrtime();
+var i = 0;
+for (var i = 0; i < 5; i++) {
+  console.log(fib(30));
+}
+var elapsed = process.hrtime(start);
+elapsed = elapsed[0] + elapsed[1] / 1000000000;
+console.log("elapsed: " + elapsed);