python bindings: export public keys
python bindings: export public keys

* Added functions for exporting public keys to gpg.core in both
  complete form and in minimised form.
* Rather than letting people need to worry about the export modes we
  are simply separating the functions as people would be more familiar
  with from the command line usage anyway.
* Functions added for Context are: ctx.key_export_minimal and
  ctx.key_export as the default or full export.

Signed-off-by: Ben McGinnes <73675debcd8a436be48ec22211dcf44fe0df0a64@adversary.org>

gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme