Script should check if target dir is writable

Hi,

Script checks if target directory exists, but it does not check permission.

Maybe something like this could do the job?
(found here: https://stackoverflow.com/a/2113511/1215291 )

```
os.access('/path/to/folder', os.W_OK)
```

Thanks for sharing your script. It is pretty useful for learning Python. I'm tired of doing such things in pure bash :)