HMAC authenticated json messages

This patch includes HMAC authenticated json messages to the serializer.
The new interface works on any json-encodable data type, and can sign it
with a private key and an optional salt. The same private key must be
used upon message loading to verify the message.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>