Refactored task end event to be Spring Boot's ApplicationReadyEvent

In testing and other use cases, the ability to keep a context open
beyond the end of a task can be useful.  This change provides the
ability to keep the context alive once a task has completed while also
explicitly shutting the context down (by default) once a task has ended.

Resolves spring-cloud/spring-cloud-task#102

spring-cloud-task-samples/timestamp/src/test/resources/application.properties

@@ -14,5 +14,5 @@
 # limitations under the License.
 #
 
-logging.level.root=DEBUG
+logging.level.org.springframework.cloud.task=DEBUG
 spring.application.name=Demo Timestamp Task