Fixed logic to select correct gpg subkey for signing/encrypting
Fixed logic to select correct gpg subkey for signing/encrypting

Some keys may have multiple subkeys that can be used for signing
or encrypting. Just because one of them may have expired or may
be revoked doesn't mean they all are. Keep looking to see if
another subkey can be used insead.

Fixes issue #88

jstedfast/gmime,GNOME/gmime,GNOME/gmime,jstedfast/gmime,GNOME/gmime,jstedfast/gmime