Bug 21786: Make balance and credits endpoints handle library_id

Bug 21786: Make balance and credits endpoints handle library_id

This patch:
- Adds the branchcode <-> library_id mapping in the controller
- Adds 'library_id' to the credit and account_line definitions
- Makes add_credit handle the library_id param, passing it to the
  underlying $account->add_credit method.

To test:
- Run:
  $ kshell
 k$ prove t/db_dependent/api/v1/patrons_accounts.t
=> FAIL: Tests fail, because library_id is not mapped
(balance) and because it is not handled when adding a credit.
- Apply this patch
- Run:
  k$ prove t/db_dependent/api/v1/patrons_accounts.t
=> SUCCESS: Tests pass!
- Sign off :-D

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Alex Arnaud <alex.arnaud@biblibre.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>