mailpipe: don't bail if a message is signed by a subkey.

mailpipe: don't bail if a message is signed by a subkey.

Also strips off a leading '0x' before matching fingerprint tails.

You should normally be using primary keys in the `pgp_key` field of
the course configuration file.  Before this commit, messages signed by
a subkey would raise WrongSignatureMessage (i.e. we were looking for a
signature by your primary key, but we only got a signature from your
subkey).  Now we look for the listed signature not only in the signing
keys but also in their primaries.

This requires pgp-mime commit:

  commit eab8b88fe3f4940a9f8285cfb8b88070cd5c0050
  Author: W. Trevor King <wking@tremily.us>
  Date:   Thu Sep 20 12:24:27 2012 -0400

    key: add pgp_mime.key wrapping gpgme-tool's KEYLIST command.