Bug 26524: Add Koha::Acquisition::Basket->orders

Bug 26524: Add Koha::Acquisition::Basket->orders

This patch adds a handy accessor for a basket's orders. It can be used
to embed orders on an API call or be used in controller scripts to
replace C4::* methods.

To test:
1. Apply this patches
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Acquisition/Basket.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>