implement WhatsApp related methods

[su-chang]

• [x] generate client instance, init client, and init client event in WhatsAppManager class #295
• [x] generate a base class named WhatsAppBase for all related classes to get / clear some attributes #288
• [x] event handler for all whatsapp-web events #294
  • [x] message event handler
  • [x] group event handler
  • [x] login related handler