Added org.ektrop.audit module

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.  

No unit tests have yet been created.  If there is interest in adding this module to the Ektorp lib, will create appropriate unit tests.