XD-863 Serialization of Spring Batch Context Objects

* Add preliminary support for a series of batch listeners
* Add sample: https://github.com/spring-projects/spring-xd-samples/tree/master/batch-notifications

Jira: https://jira.springsource.org/browse/XD-863

XD-863 Code review changes
Merge branch 'master' of github.com:SpringSource/spring-xd