mailpipe: convert to pgp-mime's new Signature verification.

mailpipe: convert to pgp-mime's new Signature verification.

This brings us up to date with the following pgp-mime commit:

  commit 7d4ff835519e6a8fa4273c364b571c2874bd31d5
  Author: W. Trevor King <wking@tremily.us>
  Date:   Thu Sep 20 10:12:38 2012 -0400

    signature: add Signature class for more Pythonic verification.

    Now verify_bytes() returns a list of `Signature`s instead of XML.
    This should be much easier for callers to handle, and it provides a
    layer of insulation between the gpgme-tool output and Python code.