Bug 15895 - Add Koha::Account module, use Koha::Account::pay internally for recordpayment

Bug 15895 - Add Koha::Account module, use Koha::Account::pay internally for recordpayment

This is the first patch in a series to unify all payment functions into
a single method.

Test Plan:
1) Apply this patch
2) prove t/db_dependent/Accounts.t
3) Test fine payment via the "Pay amount" button

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Lari Taskula <larit@student.uef.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>