Bug 20997: Add Koha::Account::Line::apply method

Bug 20997: Add Koha::Account::Line::apply method

This patch implements the ->apply() method for account lines.
It will only work on credit lines, and raise an exception otherwise.

It implements ->is_credit() as well.

To test:
- Apply this patch
- Run:
  $ kshell
 k$ prove t/db_dependent/Koha/Account/Lines.t
=> SUCCESS: Tests pass!
- Check all use cases are covered by the tests
- Sign off :-D

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>