code: public void getGroupHistory(final String groupId, final Integer maxMessages,
                                  final GroupHistoryCompletionListener completionListener) {
          getGroupHistory(groupId, maxMessages, null, completionListener);
      }
docs: Retrieve the history of messages that have been persisted for a specific group. Only those
  messages that have been marked to be persisted when sent will show up in the history. To
  retrieve messages further back in the history than right now, use the other method signature
  that allows `before` to be specified.
  
  @param groupId The groups to pull history for
  @param maxMessages The maximum number of messages per group to pull. Must be &gt;= 1
  @param completionListener The callback called when this async operation has completed