Added license to picture of day attribution box.

Made the AspectFillThumbFetcher also cache image description and categories using the request it was already making. Then made it search these categories for commonly used known licenses. Caches which license it finds using "license" and "licenseurl" keys so license info can later be displayed or used. Presently using this license data in the image of the day attribution box.
Added links to pic of the day's license and wiki pages.
Ensured updated bundled pic of the day files overwrite existing cache files.

Compared last modified dates and copied bundled file over existing cached files if they differ.
Improved the way license info is sought in categories.
Merge pull request #93 from montehurd/potdLicense

Picture of the Day license attribution