Allow GnuPG flags for anonymising keyids of encrypted messages.

 * ADD `--hidden-encrypt-to` and `--hidden-recipient` options to
   `hex_options` in gnupg._parsers._get_options_group().
 * ADD `--throw-keyids` option to `none_options` in
   gnupg._parsers._get_options_group().
 * FIXES issue #30:
   https://github.com/isislovecruft/python-gnupg/issues/30
 * NEEDS unittests still.
Merge branch 'fix/30-hidden-recipient' into develop