-------
- adapt Autocrypt header attribute names, parsing and processing to
new Level 1 spec
- add "pgpy" backend but do not activate it yet because current pgpy
versions aren ot compatible enough to gpg's crypto.
- change "sendmail" and "process-outoing" commands to not add autocrypt
headers if no account can be determined for a mail.
- add first version of "claimchain" code which py-autocrypt is to
use for its internal key management. Claimchains are an append-only
log of claims about cryptographic key material.