Replace operations with messages

[nathanielhourt]

Message type looks something like this:

/**
 * A message has a header that defines who sent it and
 * who will be processing it.  The message content is a binary blob whose
 * type is determined by 'type', which is dynamic and defined by
 * the scripting language.
 */
struct message {
  account        sender;
  account        to;
  vector<acct>   cc;
  message_type   type;
  vector<byte>   data;
};

account is a string typedef for now, will become a fixed-length string in the future.
message_type is a string typedef.