url: permalinks for book page images

[mangtronix]

Provide permalinks for page images as specified in the BookReader url specs: http://openlibrary.org/dev/docs/bookurls

[mangtronix (mang)]

Please test that all the functionality specified in the download URLs works as advertised:
http://openlibrary.org/dev/docs/bookurls#downloading

You should use www-testflip.archive.org as the test host, e.g.
http://www-testflip.archive.org/download/coloritsapplicat00andriala/page/preview_medium.jpg

Specifically you should test a variety of books to see if the image download urls work for the various cases in the bookurls doc above.

A few kinds of books to try:
* Regular Scribe books
* Protected Scribe books - the cover, title and preview images should be accessible to normal users and the other images should not
* User uploads - both with and without the sub-prefix (docs at http://openlibrary.org/dev/docs/bookurls#multibook)
* Tiff-based books, e.g. http://www.archive.org/details/nationalyiddishbookcenter
* A few other random downloads on www-testflip.archive.org, to make sure nothing else has been broken (e.g. pdf download)

Please let me know if anything isn't clear!