feat: added ACK events (#73)

Implements ack change detection, making it possible to know when messages have been delivered, read, etc.

Co-authored-by: Pedro S. Lopez <pedroslopez@me.com>

src/util/Constants.js

@@ -35,9 +35,10 @@ exports.Events = {
     MESSAGE_CREATE: 'message_create',
     MESSAGE_REVOKED_EVERYONE: 'message_revoke_everyone',
     MESSAGE_REVOKED_ME: 'message_revoke_me',
+    MESSAGE_ACK: 'message_ack',
     QR_RECEIVED: 'qr',
     DISCONNECTED: 'disconnected',
-    STATE_CHANGED: 'change_state'
+    STATE_CHANGED: 'change_state',
 };
 
 /**
@@ -89,4 +90,4 @@ exports.WAState = {
     UNLAUNCHED: 'UNLAUNCHED',
     UNPAIRED: 'UNPAIRED',
     UNPAIRED_IDLE: 'UNPAIRED_IDLE'
-};
+};