Enhancements to tasks recipe

- Fix AbstractStateMachineFactory so that it does not try
  to create transitions with null source or target. Basically we
  just skip transition source or target which doesn't belong to
  region.
- Change DefaultExtendedState to use ConcurrentHashMap so that
  there's less change to get into trouble with concurrency.
- Add statechart for tasks recipe.
- Fix various concepts in TasksHandler and its tests.
- Prepare some examples for docs.
- Fixes #74

spring-statemachine-core/src/main/java/org/springframework/statemachine/config/AbstractStateMachineFactory.java

@@ -510,7 +510,7 @@ public abstract class AbstractStateMachineFactory<S, E> extends LifecycleObjectS
 
 			if (transitionData.getKind() == TransitionKind.EXTERNAL) {
 				// TODO can we do this?
-				if (stateMap.get(source) == null && stateMap.get(target) == null) {
+				if (stateMap.get(source) == null || stateMap.get(target) == null) {
 					continue;
 				}
 				DefaultExternalTransition<S, E> transition = new DefaultExternalTransition<S, E>(stateMap.get(source),

spring-statemachine-core/src/main/java/org/springframework/statemachine/support/DefaultExtendedState.java

@@ -15,8 +15,8 @@
  */
 package org.springframework.statemachine.support;
 
-import java.util.HashMap;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 
 import org.springframework.statemachine.ExtendedState;
 
@@ -34,7 +34,7 @@ public class DefaultExtendedState implements ExtendedState {
 	 * Instantiates a new default extended state.
 	 */
 	public DefaultExtendedState() {
-		this.variables = new HashMap<Object, Object>();
+		this.variables = new ConcurrentHashMap<Object, Object>();
 	}
 
 	/**