code: def get(self, key):
          
          if isinstance(key, unicode):
              key = key.encode('utf-8')
          v = self.client.get(key)
          if v is None:
              raise KeyError("Cache key [%s] not found" % key)
          else:
              return v
docs: because memcached does not provide a function to check if a key is existed
          so here is a heck way, if the value is None, then raise Exception