Updated HeraldException content: added a Target bean

- Target.uid: The UID of the peer targeted when the exception occurred
- Target.group: Name of the targeted group of peers
- Target.uids: List of UIDs known to be in the targeted group of peers
Directory: Added get_peers_for_node(). Enhanced get_peers_for_name().

Enhanced get_peers_for_name() supports both a name or a UID.