Bug 24419: Add Koha::Suggestion->suggester accessor

Bug 24419: Add Koha::Suggestion->suggester accessor

This patch adds a ->suggester accessor to the Koha::Suggestion class. It
will return undef if no suggester, and a Koha::Patron object otherwise.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Suggestion.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>