I needed to cache some images from the internet, as well as thumbnails
of local images, for a multi-sourced image gallery. I have used this
library before so I added this class to handle the local thumbnails I
wanted to cache.  To differentiate between local files meant for this
class as opposed to FileUrlDownloader, images meant to be Thumbnailed
should be sent as a path instead of a URL. This way a mixed list of web
image urls, small local image urls, and paths to large local images that
need to be thumbnailed can be in the same set, and passed in to this in
a loop.