Gave the added content module the ability to do book jacket lookups in addition to...

Gave the added content module the ability to do book jacket lookups in addition to its current duties
in addition, book jackets are cached in memcache, which means Apache disk/mem caching won't be necessary
also added in an error checking layer where if X consecutive errors occur, all added content lookups,
except for locally cached images, are disabled until the retry timeout has passed.  These new
settings go into the added_content section of the opensrf.xml config file.

new settings:

jacket_url
retry_timeout

All that's left is to add the automatic disable capabilities to the other lookups

git-svn-id: svn://svn.open-ils.org/ILS/trunk@7737 dcc99617-32d9-48b4-a31d-7c20da2025e4