Audit trail

Provides ActiveFedora::Auditable mixin for access to Fedora audit trail information.  The mixin provides one public instance method #audit_trail which returns an instance of a private class encapsulating the audit trail information.  The #records method of the audit trail returns an array of objects with accessors representing the audit records and data points.

```ruby
obj.audit_trail.records.each do |rec|
    puts rec.id
    puts rec.process_type
    puts rec.action
    puts rec.component_id
    puts rec.responsibility
    puts rec.date
    puts rec.justification
end
```

Note: The #date method of the private FedoraAuditRecord class return a string (i.e., unparsed from the audit trail source); should this value be parsed with DateTime.strptime() ? --DCS