experiment: use PazPar2 to group related works

experiment: use PazPar2 to group related works

The approach is to use PazPar2 to search just one
target, the biblio Zebra database.  The results
of each set are merged by PazPar2 to generate a
hitlist that combines related bibs together; as an
example, if a library has the first Harry Potter
book in three languages and an audiobook format,
the hitlist should ideally return one result
for the work that includes links to the individual
bibs.

The new module C4::Search::PazPar2 implements a
simple client for PazPar2's XML-over-HTTP API.  It is
designed to be generic, and thus may end up getting
moved out of Koha to become a stand-alone CPAN module.

Signed-off-by: Chris Cormack <crc@liblime.com>
Signed-off-by: Joshua Ferraro <jmf@liblime.com>