Bug 20942: Add route to get patron's account balance

Bug 20942: Add route to get patron's account balance

This patch implements the /patrons/{patron_id}/account endpoint. It can
be used to get the patron's balance information, including outstanding
lines.

To test:
- Apply this patchset
- Run:
  $ kshell
 k$ prove t/db_dependent/api/v1/patrons_accounts.t
=> SUCCESS: tests pass!
- Make your favourite REST testing tool (RESTer on Firefox?) do:
  GET /api/v1/patrons/{patron_id}/account
to a valid patron_id. Play with Koha's UI to add credits/payments and
notice the endpoint returns the right info following the voted RFC. [1]
- Sign off :-D

[1] https://wiki.koha-community.org/wiki/Patrons_account_lines_endpoint_RFC

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>