Added x-large and xx-large thumbnail sizes

Modified to respect the XDG Base Directory Specification (http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html)

Added readonly support for shared thumbnail repositories

The subdirectories weren't a good idea. Removed them from this version.

Updated link to the MD5 implementation.

Added another sub directory level within the cache base directories to avoid too much clutter.

State not to create thumbnails for files within the thumbnail cache directory.

State when it's allowed to use thumbnails which haven't been checked for validity.

Some typo fixes.

Introduction and conclusion rewrite.

Changed handling of different thumbnail sizes.

Renamed directories.

Propose using temporary filenames to avoid problems with concurrent access.

Save thumbnails directly in the size dir without subdirs.

Added optional Thumb::Mimetype key

Give some more implementation notes.

Added "Thanks" section.

Destinction between required and optional thumbnail attributes.

Dropped distinction between global/local .thumbnail directories.

Use MD5 hashes as thumbnail filename.

Initial attempt to handle concurrent accesses by different programs.

Rewrote the "Deleting Thumbnails" section.

Rewrote this paper.

Removed distinction between low/high quality thumbnails.

Separate directory for failures.

Consider permission settings.

First public release.