Adding experimental (and optional) support of log4cxx logging.

Also, adding a sample logging configuration
diff --git a/model/sync-diff-state-container.h b/model/sync-diff-state-container.h
index 9349e76..a5e5392 100644
--- a/model/sync-diff-state-container.h
+++ b/model/sync-diff-state-container.h
@@ -45,6 +45,7 @@
   std::size_t
   operator() (DigestConstPtr digest) const
   {
+    // std::cout << "digest->getHash: " << digest->getHash () << " (" << *digest << ")" << std::endl;
     return digest->getHash ();
   }
 };
@@ -54,6 +55,7 @@
   bool
   operator() (DigestConstPtr digest1, DigestConstPtr digest2) const
   {
+    // std::cout << boost::cref(*digest1) << " == " << boost::cref(*digest2) << " : " << (*digest1 == *digest2) << std::endl;
     return *digest1 == *digest2;
   }
 };