Bug 17796: Replace Koha::Issue[s] with Koha::Checkout[s]

Bug 17796: Replace Koha::Issue[s] with Koha::Checkout[s]

Koha::Issues and Koha::Checkouts have been added to the codebase to
represent the same thing.

In ODLIS the word Issue is never used in the sense we use it. Another
problem with Issue is it has so many meaning in English (such as
problem/bug)
The word Checkout *is* in ODLIS, closer to what we use:
http://www.abc-clio.com/ODLIS/odlis_c.aspx#checkoutslip

Test plan:
  git grep Koha::Issue
should not return any occurrences and the tests must still pass

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>