Bug 22343: Add classes for handling SMTP servers

Bug 22343: Add classes for handling SMTP servers

This patch introduces classes to handle SMTP servers. It is done in a
way that a default server can be set, and then per-library ones can be
set. All should be done with Koha::SMTP::Servers methods, that take care
of overwriting library-specific configs, and handle the defaults
retrieval and setting correctly.

To test:
1. Apply this patches
2. Run:
   $ kshell
   $ perl installer/data/mysql/updatedatabase.pl
=> SUCCESS: Atomic update is ok, smtp_servers table created
3. Run:
  k$ prove t/db_dependent/Koha/SMTP/
=> SUCCESS: Tests pass!
4. Sign off :-D

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>