Bug 20990: Add Koha::Account->outstanding_credits

Bug 20990: Add Koha::Account->outstanding_credits

This patch adds a method that retrieves (for a patron's account) the
outstanding credits (i.e. those that haven't been applied to any debit.

To test:
- Apply this patches
- 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>