feat(stop): ability to stop bot from listening on updates

resolve #12

lib/functions/poll.js

@@ -5,6 +5,7 @@ export default function poll(bot) {
     }
     bot.emit('update', response.result);
 
+    if (bot._stop) return null;
     return poll(bot);
   });
 }

lib/functions/webhook.js

@@ -12,7 +12,7 @@ export default function webhook(options = {}, bot) {
 
   return bot.api.setWebhook(options.url).then(() => {
 
-    https.createServer(options.server, (req, res) => {
+    bot._webhookServer = https.createServer(options.server, (req, res) => {
       return getBody(req).then(data => {
         bot.emit('update', qs.parse(data).result);