Bug 21896: Add Koha::Account::normalize_balance

Bug 21896: Add Koha::Account::normalize_balance

This patch adds the normalize_balance() method to Koha::Account. Its
purpose is to apply outstanding credits (i.e. manual ones ore remaining
amounts like in the case of refunds) to outstanding debts.

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

Signed-off-by: Christopher Brannon <cbrannon@cdalibrary.org>
Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>