windows fix caused error on unix systems, this should fix both

code/application/requestHandlers.js

@@ -31,13 +31,15 @@ function upload(response, request) {
   console.log("about to parse");
   form.parse(request, function(error, fields, files) {
     console.log("parsing done");
-
-    /*
-     * Some systems [Windows] raise an error when you attempt to rename new file into one that already exists.
-     * This call deletes the previous .PNG image prior to renaming the new one in its place.
-    */
-    fs.unlinkSync("/tmp/test.png");
-    fs.renameSync(files.upload.path, "/tmp/test.png");
+
+    /* Possible error on Windows systems:
+       tried to rename to an already existing file */
+    fs.rename(files.upload.path, "/tmp/test.png", function(err) {
+      if (err) {
+        fs.unlink("/tmp/test.png");
+        fs.rename(files.upload.path, "/tmp/test.png");
+      }
+    });
     response.writeHead(200, {"Content-Type": "text/html"});
     response.write("received image:<br/>");
     response.write("<img src='/show' />");