add an optional category to be inserted into the path

This lets us group attachments (for a particular customer or event).

Given this configuration:

```
ImageSchema.plugin(attachments, {
    directory: "/home/rngadam/letsface/src/prototype/public/images",
    storage : {
        providerName: 'localfs',
        options : {
          removePrefix: "/home/rngadam/letsface/src/prototype/public"  
        }        
    },
    properties: {
        image: {
            styles: {
                original: {
                    // keep the original file
                },
                thumb: {
                    thumbnail: '100x100^',
                    gravity: 'center',
                    extent: '100x100',
                    '$format': 'jpg'
                },
                detail: {
                    resize: '400x400>',
                    '$format': 'jpg'
                }
            }
        }
    }
});
```

output of image property is:

```
 image: 
   { detail: 
      { depth: 8,
        format: 'JPEG',
        oname: 'displayed.png',
        mtime: Tue Oct 22 2013 14:27:03 GMT+0800 (CST),
        ctime: Tue Oct 22 2013 14:27:03 GMT+0800 (CST),
        size: 15957,
        path: '/home/rngadam/letsface/src/prototype/public/images/zO0vt4yIo/detail/Sbz4kipeRM-detail.jpg',
        defaultUrl: '/images/zO0vt4yIo/detail/Sbz4kipeRM-detail.jpg',
        dims: [Object] },
     thumb: 
      { depth: 8,
        format: 'JPEG',
        oname: 'displayed.png',
        mtime: Tue Oct 22 2013 14:27:03 GMT+0800 (CST),
        ctime: Tue Oct 22 2013 14:27:03 GMT+0800 (CST),
        size: 2952,
        path: '/home/rngadam/letsface/src/prototype/public/images/zO0vt4yIo/thumb/Sbz4kipeRM-thumb.jpg',
        defaultUrl: '/images/zO0vt4yIo/thumb/Sbz4kipeRM-thumb.jpg',
        dims: [Object] },
     original: 
      { depth: 8,
        format: 'PNG',
        oname: 'displayed.png',
        mtime: Tue Oct 22 2013 14:27:01 GMT+0800 (CST),
        ctime: Tue Oct 22 2013 14:27:01 GMT+0800 (CST),
        size: 77825,
        path: '/home/rngadam/letsface/src/prototype/public/images/zO0vt4yIo/original/Sbz4kipeRM-original.png',
        defaultUrl: '/images/zO0vt4yIo/original/Sbz4kipeRM-original.png',
        dims: [Object] } },
```