DATAJPA-367 - Issue error for missing spring-aspects.jar if auditing is enabled in xml config.

As the auditing feature requires spring-aspects.jar on the classpath we issue now check whether the jar is present - if not we issue an appropriate error message. Added appropriate note to the JPA reference doc. Added test case to AuditingBeanDefinitionParserTests to verify that a XMLParserError message is emitted if the required class from spring-aspects is not on the class path.

Original pull request: #34.