core: added get method to PersistentDictionary

src/memory/persistent.py

@@ -104,6 +104,9 @@ class PersistentDict:
             self.storage.delIndPrivate(self.namespace, key, self.profile)
         return self._cache.__delitem__(key)
 
+    def get(self, key, default=None):
+        return self._cache.get(key, default)
+
     def force(self, name):
         """Force saving of an attribute to storage
         @return: deferred fired when data is actually saved"""