#37 Option to store thumbs separately

[Sebastian Schuberth]

As there are cases where "pictures_dir" may point to a read-only location, or users simply do not want to store thumbnails at the same location as thumbnails, I'd like to suggest to introduce a "thumbs_dir" (or "thumbs_root") option. If that option is set, the directory prefix where thumbnails are stored should simply be changed so that they are not stored at

$pictures_dir/path/to/your/images/.thumbs

but

$thumbs_dir/path/to/your/images/.thumbs

That is, you have basically the same directory structure under "pictures_dir" and "thumbs_dir", except the ".thumbs" directories, which are only present in the latter.