Fix gulp on Windows

I tested it on Windows XP and everything seems to be working, but unfortunately I'm unable to check it on other versions of Windows (Vista/7/8/10).
![screen shot 2014-12-22 at 8 05 07 pm](https://cloud.githubusercontent.com/assets/47926/5528262/33fa7e62-8a17-11e4-9f1f-d3bc727786f9.png)

Because this one goes too deep.. maybe something like this:

```
if not os.path.exists(file_dir):
  return
if os.path.isdir(file_dir):
  ...
```