Support for breeding

One party initiates the breeding; the other party exerts most of the
energy if its breeding key matches. It is that party's responsibility to
set up the newborn agent's connection (but the father can pass it an
arbitrary message). Newborn is spawned immediately but can be
renegotiated on first connect.

connection.h

@@ -28,6 +28,7 @@ public:
 	}
 
 	void senses(int tick_id, class agent &);
+	void bred(int agent_id, std::string &info);
 	void actions(class agent *);
 
 	void cancel(void);