Add support for new webhook events

[remixz]

Here are the ones we're missing now:

• https://developers.facebook.com/docs/messenger-platform/webhook-reference/account-linking
• https://developers.facebook.com/docs/messenger-platform/webhook-reference/message-read
• https://developers.facebook.com/docs/messenger-platform/webhook-reference/message-echo

Should be a pretty simple matter of just adding them to the _handleMessage function (https://github.com/remixz/messenger-bot/blob/master/index.js#L95), and then adding tests.

[remixz]

Actually, we already have basic support for the message echo event, as it'll be captured by our message event callback. However, because I think most people would rather not handle their own messages as incoming messages, I think we should create a new event by checking if the is_echo param is on the incoming payload, and then delegate based on that.

[eXeDK]

Have a look at the #26 pull request, should do this. Sadly I didn't see your issues before I wrote the code.