Bug 13098: Add Koha::Account::Offsets->total

Bug 13098: Add Koha::Account::Offsets->total

This patch introduces this trivial method for returning the sum0 result
of the amount columns of a Koha::Account::Offsets set.

To test:
- Apply this patch
- Run:
  $ kshell
 k$ prove t/db_dependent/Koha/Account/Offsets.t
- Sign off :-D

Followed test plan, patch worked as described
Signed-off-by: Alex Buckley <alexbuckley@catalyst.net.nz>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>