Bug 17740: Add the Koha::Patron->holds method

Bug 17740: Add the Koha::Patron->holds method

The goal of this method is to replace
C4::Reserves::GetReservesFromBorrowernumber but could be reused for
something else, that's why it has its own bug report.

Test plan:
  prove t/db_dependent/Koha/Patrons.t
should return green

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Rebased. Edited two test descriptions.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>