Use GnuPG 2 if available

* debian/control (Package: mandos/Depends): Change "gnupg (<< 2)" to
                                            "gnupg".
  (Package: mandos-client/Depends): Remove "gnupg (<< 2)"; it will be
                                    depended on by libgpgme11, which
                                    will be added automatically by
                                    ${shlibs:Depends}.
* mandos (PGPEngine.__init__): Try to run "gpgconf" and set 'self.gpg'
                               to any binary found.  Also change
                               "--home" to "--homedir".
  (PGPEngine.encrypt, PGPEngine.decrypt): Use the 'self.gpg' attribute
                                          instead of "gpg".