Bug 17778 - Make "Earliest Registered Date" in OAI dynamic

Bug 17778 - Make "Earliest Registered Date" in OAI dynamic

This was hardcoded to 0001-01-01.

To test:
- Make sure you have a couple of records, with different timestamps
- Enable OAI-PMH
- Check http://localhost:2201/cgi-bin/koha/oai.pl?verb=Identify
  and verify that "Earliest Registered Date" is 0001-01-01
- Apply the patch
- Re-check the "Earliest Registered Date" and verify that it is now
  the lowest timestamp in your biblio table
- Manipulate the timestamps and verify that "Earliest Registered Date"
  changes accordingly
- Sign off!

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit ff711c48cfa02c7b6ab09fe22f699a872420db8d)
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>