Bug 13496: add a method for the templates to print Koha version as desired

Bug 13496: add a method for the templates to print Koha version as desired

This patch introduces a simple template plugin function that returns
the Koha version number in different suitable formats.

It introduces unit tests for the new function. They are run with

To test:
- Apply the patch
- Run
  $ prove t/Koha_Template_Plugin_Koha.t
=> SUCCESS: the tests pass.
- Sign off :-D

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>