Multiple Signers did not work with gpgme 2.0.2

Exchanged :signers with :signer, this seems to allow to create a
signature based on account or from.

Previously always the default or first private key had been used.

But I haven't communicated with the developers of gpgme.

Signed-off-by: Gaute Hope <eg@gaute.vetsj.com>
mbox: remove deperacted enumerator.new
Merge branch 'mbox-rm-enum-new' of https://github.com/gauteh/sup into develop
Encryption did not fail if no key for recipient was found.

The problem is rooted in the simplicity of GPGME:Crypto and GPGME:Ctx
encrypt, which both have no problem with missing keys (only with invalid
ones)

Fix verifies that for any pattern in the To: field there exists a key
(found by Ctx.keys).

This is a little clumsy. I would like to
a) have a key selection or search for missing keys
b) have a user verify the choice of encryption keys before sending them
   (unless configured otherwise)