Add diff function to Audit reader, AuditReaderInterface in admin-bundle ...

In order to use the diff function, the default `AuditReader` should actually support the diff function. This function is automatically supported, since it directly accesses the AuditReader of the EntityAudit repository: https://github.com/simplethings/EntityAudit/blob/master/src/SimpleThings/EntityAudit/AuditReader.php