Support for cancelling pending events

Implement client.cancelPendingEvent which will cancel queued or not_sent events

lib/models/event.js

@@ -36,6 +36,9 @@ module.exports.EventStatus = {
     /** The event has been sent to the server, but we have not yet received the
      * echo. */
     SENT: "sent",
+
+    /** The event was cancelled before it was successfully sent. */
+    CANCELLED: "cancelled",
 };
 
 /**