Bug 22343: Revamped Koha::Email class

Bug 22343: Revamped Koha::Email class

This patch completely rewrites the Koha::Email class, inheriting from
Email::Stuffer. The latter suits well the use by Email::Sender, which is
to replace Mail::Sendmail on this patchset.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/Koha/Email.t
=> SUCCESS: Tests pass!
3. Verify all conditional codepaths are covered
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>