Add local caching support.

Add support for caching generated thumbnails locally.

If you set `cache_dir` to the directory in which cached thumbnails should be
stored, and `cache_url_stub` to the URL to this directory, then URLs returned
will be the URLs to images cached in that directory for you.
Include requested size in hashed filename.

Otherwise, if you change the size you want, you'd still get the cached file of
the old size.
Remove warning
Bump version to 0.04
Update Changes
Update prereqs
Add META_MERGE for GitHub URLs to Makefile.PL
Add example.

Might be useful to users, and boosts CPANTS score :)
Add example/getthumb.pl to MANIFEST
Bump version to 0.05 and update Changes