Bug 20978: Add Koha::Account::add_credit

Bug 20978: Add Koha::Account::add_credit

This patch adds:
- Koha::Account::add_credit
- A single source of truth for accounttype and offset_type values

To test:
- Apply the first patch
- Run:
  $ kshell
 k$ prove t/db_dependent/Koha/Account.t
=> FAIL: Tests fail!
- Apply this patch
- Run:
 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>