Payment APIs Refinements

[JasonBoy]

• ✨ Add HMAC-SHA256 sign type for some apis do not support MD5
• ✨ A simple demo implementation for Order class in a general online shop system
• ✨ Add API decryptRefundNotifyResult to decrypt wechat refund notify result data
• ✨ Add API queryComments to query user comment for their payment order
• ✨ Add API downloadFundFlow to download merchant fund flows
• 🔧 Download API will resolved/rejected with:

{
  //an error object, rejected
  error: new Error(),
  //error msg, , rejected
  msg: '',
  //resolved with download stream,
  //can be piped to other writable stream, e.g: result.data.pipe(fs.createWritableStream('./bill.txt'))
  data: Stream,
  //sign info from response header
  digest: 'SHA=ec45d7c24492dcd62d92472b0f2816c8d9a2d773',
}

• 🐛 Other Fixes and Improvements
• 🙃 Demo page updates: