Pigeon::Message was holding on to too many drafting responsibilities.
It also was possible for the local identity to inadvertantly author
malformed messages. This is because local messages were not passed
through the ::Lexer / ::Parser and thereby did not receive the same
scrutiny of remote messages.
To avoid security problems later, and for additional security,
I will only allow messages to be saved *after* passing through the
lexer/parser. This means moving much of Pigeon::Message's logic
into Pigeon::Draft.
