Add parallel performance test to demonstrate effect of waiting for a worker thread

tests/parallel.js

@@ -0,0 +1,29 @@
+var sharp = require("../index");
+var fs = require("fs");
+var assert = require("assert");
+var async = require("async");
+
+var inputJpg = __dirname + "/2569067123_aca715a2ee_o.jpg"; // http://www.flickr.com/photos/grizdave/2569067123/
+var width = 720;
+var height = 480;
+
+async.mapSeries([1, 1, 2, 4, 8, 16, 32, 64, 128], function(parallelism, next) {
+  var start = new Date().getTime();
+  async.times(parallelism,
+    function(id, callback) {
+      sharp.resize(inputJpg, sharp.buffer.jpeg, width, height, function(err, buffer) {
+        buffer = null;
+        callback(err, new Date().getTime() - start);
+      });
+    },
+    function(err, ids) {
+      assert(!err);
+      assert(ids.length === parallelism);
+      var mean = ids.reduce(function(a, b) {
+        return a + b;
+      }) / ids.length;
+      console.log(parallelism + " parallel calls: fastest=" + ids[0] + "ms slowest=" + ids[ids.length - 1] + "ms mean=" + mean + "ms");
+      next();
+    }
+  );
+}, function() {});