Bug 21082: Add new admin page for overdrive

The 'authname' field required for OverDrive can differ per branch.
This patch adds Koha Objects for dealing with OD info and submitting
authnames per branch. The description is left open so future branch info
can be added.

To test:
1 - prove -v t/db_dependent/Koha/Library/OverDriveInfos.t
2 - visit cgi-bin/koha/admin/overdrive.pl
3 - Add some authnames for various branches
4 - Verify data saves correctly

Signed-off-by: Sandy Allgood <sandy.allgood@citruslibraries.org>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>