bug: 2272 - remove warning from C4::Koha::getitemtypeimagedir

bug: 2272 - remove warning from C4::Koha::getitemtypeimagedir

I changed getitemtypeimagedir to set a default on its argument so that it would not complain if not passed 'opac'.
I improved the documentation on the method.
I edited the t/icondirecotries.t test script to explicitly pass an argument to both getitemtypeimagedir calls.
 - and I adjusted one line of whitespace to make similar things look similar
I added a test module for C4::Koha
I added a test module for C4::Koha::getitemtypeimagedir.

Signed-off-by: Joshua Ferraro <jmf@liblime.com>