First pass at a PDF downloading/etag/sha system.

Will now download PDFs and version them. If the PDF has already
been downloaded, will check the ETag on the server and only re-download
if changed. Force a re-download with the 'everything' command. Can
be used with 'check everything' (re-download first page of PDFs), or
'check archive everything' (re-download all pages of PDFs, expensive).