Added the ability to implement TaskExecutionListener functionality without
implementing the TaskExecutionListener interface.
resolvesspring-cloud/spring-cloud-task#95
TaskLifecycleListener will set a TaskExecution's exitCode to null upon taskStart in the repository.
Only at the end of the execution will it store the exitCode.
resolvesspring-cloud/spring-cloud-task#82
This is the initial cut of the reference documentation for Spring Cloud
Task.
Resolvesspring-cloud/spring-cloud-task#49
Updates per code review
use mvnw instead of mvn in example