bug 2000 - add C4::Circulation::GetBranchBorrowerCircRule

bug 2000 - add C4::Circulation::GetBranchBorrowerCircRule

This routine retrieves the branch/patron category circulation
rules for a given branch and patron category.  The return
value is a hashref containing the following key:

maxissueqty - maximum number of loans across all item types

This will first check for a specific branch and
category match from branch_borrower_circ_rules.

If no rule is found, it will then check default_branch_circ_rules
(same branch, default category).  If no rule is found,
it will then check default_borrower_circ_rules (default
branch, same category), then failing that, default_circ_rules
(default branch, default category).

If no rule has been found in the database, it will default to
the built in rule:

maxissueqty - undef

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