Adding button to deploy to heroku

To solve the problem of ports I made a condition to use the ENV `PORT` value returned by most clounds otherwise use the default port 80.

I currently use this implementation without any problems.

closes #88 and closes #114

app.json

@@ -0,0 +1,5 @@
+{
+    "name": "localtunnel-server",
+    "description": "localtunnel exposes your localhost to the world for easy testing and sharing!",
+    "repository": "https://github.com/localtunnel/server"
+}