Bug 9580 Cover images from Coce, a remote image URL cache

Bug 9580 Cover images from Coce, a remote image URL cache

Add to Koha support for displaying Book cover requested to Coce, a
remote image URL cache:

  https://github.com/fredericd/coce

With this enhancement, cover images are not fetched directly from
Amazon, Google, and so on. Their URL are requested via a web service to
Coce which manages a cache of URLs.

Three cover image providers are now available: Google Books (gb), Amazon
(aws), and Open Library (ol). Two system preferences enable this service:

  - CoceHost - URL of Coce server
  - CoceProviders - Ordered list of requested providers. For example
    gb,aws,ol, will return the first available URL from those providers.

Several advantages to this architecture:

  - If a book cover is not available from a provider, but is available
    from another one, Koha will be able to display a book cover, which
    isn't the case now
  - Since URLs are cached, it isn't necessary for each book cover to
    request, again and again, the provider, and several of them if
    necessary.
  - Amazon book covers are retrieved with Amazon Product Advertising
    API, which means that more covers are retrieved (ISBN13).

Test plan:

- Apply this patch, and test with 'Bootstrap' themes
- You can verify that nothing has changed on OPAC result and detail
  page, since new syspref haven't been filled
- Install Code: https://github.com/fredericd/coce
  Or ask me directly for the URL of a Coce server
- In sysprefs, tab Enhanced content, modify:
  CoceHost -- enter the URL of your Coce server
  CoceProviders -- fill with: gb,aws,ol
- Do a search. On result page, and detail page, you will see cover
  images originating from the 3 providers: fly over the image to see its
  URL. Try to compare with a Koha with just GoogleJacket or Amazon cover
  to confirm that you have more images. Verify that it's quick, and even
  quicker for cached images URLs.
- Check that Browse Shelf functionnality works properly.
- The ID sent to Coce is EAN or ISBN. Try with various type of biblios. DVD
  have often EAN, but no ISBN.
- You can try with those sysprefs:
    CoceProviders: aws,gb
    CoceHost: http://coce.tamil.fr:8080
  and this EAN (or ISBN): 3333297517744
  => OPACAmazonCoverImages enabled doesn't display a cover because, it's a DVD
  => Coce find and display the DVD cover.

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Amended patch: replaced 1 tab with spaces in C4/Shelfbrowser.pm

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>