Add distibuted action

- New module spring-statemachine-cluster which is
  based on spring-cloud-cluster to provide leader
  election.
- Ensemble now has a concept of a leader if implementation
  supports it.
- New DistributedLeaderAction can use leader info to execute
  action only on a leader.
- Tweak web sample with these new concepts.
- Fixes #176

spring-statemachine-zookeeper/src/test/java/org/springframework/statemachine/zookeeper/ZookeeperStateMachineEnsembleTests.java

@@ -671,6 +671,14 @@ public class ZookeeperStateMachineEnsembleTests extends AbstractZookeeperTests {
 			errorLatch.countDown();
 		}
 
+		@Override
+		public void ensembleLeaderGranted(StateMachine<String, String> stateMachine) {
+		}
+
+		@Override
+		public void ensembleLeaderRevoked(StateMachine<String, String> stateMachine) {
+		}
+
 		public void reset(int c1, int c2) {
 			reset(c1, c2, 0);
 		}