DATAJPA-265 - Support for auditing configuration via JavaConfig.

Introduces the necessary infrastructure to configure auditing with JPA via JavaConfig using @EnableJpaAuditing.

Original pull request: #50.
DATAJPA-265 - Improve implementation of AuditingBeanFactoryPostProcessor.

Polished the implementation of AuditingBeanFactoryPostProcessor to selectively add depends-on clauses to all bean definitions that will result in EntityManagerFactory instances eventually. Added unit tests to verify intended behavior for Java based configuration. Polished newly integrated test cases.

Removed obsolete code from AuditingEntityListener. Added configuration sample snippet. Polished reference documentation.

Original pull request: #50.