Bug 20946: Add Koha::Account::outstanding_debits

Bug 20946: Add Koha::Account::outstanding_debits

This patch adds a handy method that returns the total for outstanding
debits (i.e. those that haven't been canceled with credits), and the
corresponding Koha::Account::Line objects.

Unit tests are added.

To test:
- Apply this patch
- Run:
  $ kshell
 k$ prove t/db_dependent/Koha/Account.t
=> SUCCESS: tests pass!
- Sign off :-D

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>