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/main/java/org/springframework/statemachine/zookeeper/ZookeeperStateMachineEnsemble.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2015 the original author or authors.
+ * Copyright 2015-2016 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -167,6 +167,11 @@ public class ZookeeperStateMachineEnsemble<S, E> extends StateMachineEnsembleObj
 		}
 	}
 
+	@Override
+	public StateMachine<S, E> getLeader() {
+		return null;
+	}
+
 	private void joinQueued() {
 		StateMachine<S, E> stateMachine = null;
 		synchronized (joinLock) {

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);
 		}