CBUC: Koha::Template::Plugin::Covers::CBUC introduced

This patch introduces a new template plugin for building URL's for fetching
cover images from the CBUC repository.

See perldoc Koha/Template/Plugin/Covers/CBUC.pm for further details.

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