Images resource should be able to retrieve only specific fields

The images resource currently returns a lot of image data. The resource should support a `fields` parameter, containing a comma separated list of fields to include in the output.

`http://imbo/users/user/images.json?fields=size,height,width` 

would return something like this:

```
[
    {
        "size": 123456,
        "height": 768,
        "width": 1024
    },
    {
        "size": 234567,
        "height": 1000,
        "width": 3000
    }
]
```