Added org.ektrop.audit module

[benze]

This is an initial commit of an Audit module. It requires AspectJ to weave in the code around the Ektorp libs. It advises the update & commit methods of the CouchDBConnector interface to update the created/modified fields. In order to persist any createdBy/updatedBy information, a AuditorAware implementation must be configured in the AuditAspect.

Documents to be audited can either manually implement the AuditableBean interface, or annotated the class with an @Auditable annotation. AspectJ will automatically extend the class to implement the AuditableBean interface and append the necessary fields.

Module was heavily influenced by spring-data auditing module, but significantly simplified.

I added a unit test AuditTest to give a quick indication of how it works.