Folder Names with Umlaut are not Supported

With this code you will end up with 
ASCII control chars as path:

```
var zip = new JSZip();
zip.file("Hello.txt", "Hello World\n");
var img = zip.folder("öäü");
img.file("smile.gif", imgData, {base64: true});
var content = zip.generate();
location.href="data:application/zip;base64,"+content;
```
