Bug 23770: Add Koha::Object(s)->to_api method

Bug 23770: Add Koha::Object(s)->to_api method

Generic method that changes the attribute names of an object in the
presence of the to_api_mapping method. It otherwise falls back to
returning the TO_JSON output.

This is WIP submitted early for scrutiny. Tests and example usage in an
API controller coming.

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>