Bug 26595: Add Koha::Library->smtp_server_info

Bug 26595: Add Koha::Library->smtp_server_info

This patch adds a method for retrieving brief information about the
linked (or not) SMTP server for embedding on API requests.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Library.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>