DATAJPA-265 - Make it possible to configure auditing with Java Config.

Added JpaAuditingRegistrar, JPA specific implementation for configuring auditing via Java Config. To enable auditing annotate your @Configuration classes with @EnableJpaAuditing, providing atleast a reference to a AuditorAware. @EnableJpaAuditing automatically imports SpringConfiguredConfiguration, which enables dependency-injection on objects not managed by spring. Also provided integration tests.

Support for enabling auditing via java config. Feel free to polish the code a bit :)