AthMessaging: add constructor with MessageSvc auto-retrieval

Add constructor that takes care of retrieving the MessageSvc. This simplifies the standard-use case from:

#include "AthenaKernel/getMessageSvc.h"

MyClass::MyClass() :
  AthMessaging(Athena::getMessageSvc(), "MyClass")

to

MyClass::MyClass() :
  AthMessaging("MyClass")

If needed, caching of the MessageSvc could be implemented as well.

cc @ssnyder @leggett