DATAMONGO-792 - Add support to configure auditing via JavaConfig.

Introduces the necessary infrastructure in form of MongoAuditingRegistrar to configure auditing with MongoDB via Annotation config. The MongoDB auditing feature can be enabled by annotating a configuration class with the EnableMongoAuditing annotation. Added section to reference documentation.

Original pull request: #94.
DATAMONGO-792 - Polishing of JavaConfig support for auditing.

Original pull request: #94.