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

I just created the following pull request, which will need this pull request: 
https://github.com/sonata-project/SonataAdminBundle/pull/2266

If you want this pull request and you want me to make a seperate PR on the admin-bundle repository with only the `AuditReaderInterface` modification, please let me know.