Accept existing Vignette URLs

Accept existing Vignette URLs for getThumbURL(), in order to change the
mode of an existing thumbnail or convert a full-sized image to a
thumbnail.
We can get rid of the `/`-s and `&`-s by treating the URL as two arrays, before and after `?` and then join the elements of the two arrays using the separators and having `?` in between.

Something like:
````javascript
var url = [
  domain,
  'path1',
  'path2'
].join('/'),
query = [
  'cb='+cacheBuster
];

if (bla) {
  query.push('bla=bla')
}

return url + '?' + query.join('&');
```

What do you think ?