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>
Merge branch 'develop' into maildir-root