Octopress support and selectable scaling

Noticed that using gallery generator in subdirectories (Like source/ in octopress), makes the thumbs disappear after generation, as the path is incorrectly set for subdirectories, in static_files. This should fix the issue for octopress.

Note: This could probably be made more general, to fix issues for other folders than source.

Also, I added a feature, allowing you to set scale method in config, to allow for "fill-scaling", as I use resize_to_fill to generate square thumbnails.

Thanks for a great, tiny plugin :)