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.

Note that this depends on DATACMNS-389 (spring-data-commons #53) to be applied.
DATAMONGO-792 - Add support to configure Auditing via JavaConfig.

Added section to reference documentation.