nodes can heartbeat for all of their instances

also change Procfiles to list only one node; use -c node=X for X nodes

to try the heartbeating: foreman start -f Procfile.heartbeats -c node=10

kill a node (kill PID) and see that after 30 seconds the other nodes recover
his instances

the default Procfile does the crashing-instances demo instead, because it's
more fun